Charged droplet sprayers

ABSTRACT

Charged droplet spray is formed from a solution with all or a portion of the charged droplet spray current generated from reduction or oxidation (redox) reactions occurring on surfaces removed from the first or sample solution flow path. In one embodiment of the invention, two solution flow channels are separated by a semipermeable membrane. A first or sample solution flowing through the first solution flow channel exchanges cation or anion charged species through the semipermeable membrane with a second solution or gas flowing through the second flow channel. Charge exchange is driven by the electric field applied at the charged droplet sprayer sample solution outlet. Redox reactions occur at an electrode surface in contact with the second solution. The invention increases the control and range of the Electrospray ionization process during ES/MS operation. Alternative embodiments of the invention provide for conducting redox reactions on conductive surfaces removed from the first or sample solution flow path but not separated by semipermeable membranes.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application No.60/573,666, filed on May 21, 2004, the disclosure of which isincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The invention relates to the production of charged liquid droplet spraysgenerated in Electrospray ionization sources interfaced to massspectrometers.

BACKGROUND OF THE INVENTION

Sprays of charged liquid droplets can be produced through Electrosprayand pneumatic nebulization or ultrasonic nebulization in the presence ofan electric field. The mechanisms of ion production from unassistedElectrospray ionization have been described by Karbarle, P., J. MassSpectrom. 35, 804-817 (2000)[1], and Karbarle, P. and Ho, Y.“Electrospray Ionization Mass Spectrometry”, Edited by Richard Cole,Chapter 1, 3-63, 1997[2]. The oxidation and reduction chemical reactionsthat occur, or that can be induced to occur, on conductive surfaceslocated in sample solution flow channels prior to or during the chargeddroplet formation process in Electrospray have been described by VanBerkel, G. L. “Electrospray Ionization Mass Spectrometry”, Edited byRichard Cole, Chapter 2, 65-105, 1997[3], Van Berkel, G. J., J. Am. Soc.Mass Spectrom. 2000, 11, 951-960[4] and Van Berkel, G. J., Asano, K. G.and Kertesz, V., Anal. Chem. 2002, 74, 5047-5056[5]. Promotion ofoxidation or reduction of sample species on conductive surfaces duringElectrospray ionization followed by mass spectrometric analysis can be auseful tool to enhance the sensitivity or aid in determining thestructure of specific sample species. The production of ion species insample solutions through reduction/oxidation reactions on surfaces inthe first solution flow channel with solutions retaining a total netneutral charge prior to Electrospraying for mass spectrometrometricanalysis has been reported by Hackett et. al., U.S. Pat. No.5,869,832[6]. Commercially available products are available from ESAInc., Chelmsford, Mass., that promote electrochemical reactions onsurfaces in the sample solution flow path by applying voltages acrosselectrodes extending into the sample solution flow. Specific electrodematerials have been explored to control analyte oxidation in samplesolutions prior to Electrospraying [5]. Split flow fractionationtechniques used in conjunction with Electrospray ionization have beendescribed by Van Berkel, U.S. Pat. No. 6,677,593 B1 [7] where electricor magnetic fields are applied across a sample solution in a flow pathusing two electrodes positioned on opposite sides of a sample solutionflow path in contact with the solution flow to separate positive andnegative ions into separate sample solution flow streams prior tocharged droplet spraying. Van Berkel describes charged droplet sprayingfrom such devices even without the presence of an external electricfield applied at sample solution channel exit tips. MultipleElectrospray tips have been configured from a single sample solutionflow channel by Kostianen and Bruins, Rapid Comm. in MS, Vol. 8, 549-558(1994)[8]. Simultaneous Electrospraying of a sample solution frompositive and negative sprays partitions the sample species in a mannerthat may not be readily predictable or controlled.

Neutral and charged species have been exchanged across membranes,transferred into and/or removed from sample solution flows to reduce oreliminate selected species in exchange for other selected species in asample solution prior to Electrospraying. Acid and/or saltconcentrations in a sample solution have been reduced by exchange acrossspecies specific semipermeable membranes prior to Electrospraying.Charged or neutral species are exchanged between a sample solution and asecond solution through a semipermeable membrane driven by concentrationgradients or electric fields maintained across such membranes whileretaining an electrically neutral sample solution. In such deviceselectrodes are positioned in the first and second flow channels incontact with the sample solution and the second solution. Chargedspecies in the sample and second solutions are neutralized through redoxreactions occurring on the first and second flow channel electrodeconductive surfaces resulting in a net neutral sample solution flowexiting these membrane devices. Such devices have been described and aresold by Dionex Corporation. The electric field is maintained across themembrane in these devices by applying a voltage difference betweenelectrodes positioned in the first and second solution flow channels oneither side of the membrane. The electric field applied across theseelectrodes drives the charged species across the membrane between thesample solution and second solutions. The electric field applied acrossthe membrane in these devices is configured upstream and operatedindependent of a second electric field formed in the Electrosprayprocess if these devices are interfaced to an Electrospray ion sourcethrough a connecting flow tube. The charged droplet spray currentproduced in these devices interfaced to an Electrospray probe isgenerated from redox reactions occurring at conductive surfaces locatedin the sample solution flow channel.

The present invention eliminates the occurrence of oxidation orreduction reactions on conductive surfaces in the sample solution flowchannel during Electrospray Ionization (ES) while providing control ofthe total Electrospray current generated during the charged dropletformation process. The total Electrospray current has a direct impact onthe size distribution of the charged droplets produced. In oneembodiment of the invention, a sample solution flow channel is separatedfrom a second solution or gas phase flow channel by a semipermeablemembrane. The solution or gas composition flowing through the secondflow channel can be varied as a step function or gradient duringElectrospraying. In charged droplet sprayer embodiments configuredaccording to the invention, the Electrospray field present at the samplesolution spray tip during Electrospray is the only electric fielddriving charged species formation in the sample solution and secondsolution flow channels. Van Berkel, et. al. [5] describe the use of acellulose ester 5000 Da molecular mass cutoff membrane membrane coveringan electrode surface to prevent redox reactions of sample molecules onthe electrode surface during the Electrospraying. The electrode ismaintained at a kilovolt potential during Electrospraying, with anupstream grounded electrode positioned the sample solution flow path. Nosecond solution is used behind the membrane in the reported Electrosprayapparatus and no current measurement was taken on the groundedconductive surface in the sample flow path during Electrosprayionization to determine the extent of redox reactions occurring at thegrounded electrode surface in the sample solution flow path. Noexplanation is given by the authors as to how the electrical contact iscompleted between the sample solution and the electrode through themembrane but it is likely that the sample solution wetting the membraneforms the electrical contact with the electrode maintained at kilovoltpotentials during Electrospray ionization.

Severs, J. C., Harms, A. C., and Smith, R. D., Rapid Communications inMass Spectrometry, Vol. 10, 1175-1178 (1996) and Severs, J. C. [9] andSmith, R. D., Anal. Chem. 1997, 69, 2154-2158 [10] describe a capillaryelectrophoresis (CE) Electrospray interface with a mass spectrometer(MS) in which a polysulfone dialysis membrane with a molecular weightcutoff of 10,000 Da separates the capillary electrophoresis solutionfrom a second electrolyte solution in contact with a CE column exitelectrode. In the CE/ES/MS interface described, the total Electrospraycurrent is a small fraction of the total CE current flowing to the CEcolumn exit electrode surface. In the CE runs reported, a +30 kVpotential was maintained at the CE column entrance. In positive ion modeCE/ES operation reported, reduction occurs at the CE column exitelectrode maintained at +1.6 kV as electrons pass from the electrodeinto the second electrolyte solution. During this CE/ES operationdescribed, net positive charge transfers from the CE column solutioninto the second electrolyte solution through the membrane duringpositive Electrospray ionization. The net positive charge for chargeddroplet production in Electrospray appears to be supplied by a smallportion of the electrophoretic charge moving from entrance to exitthrough the CE column driven by the 30 kV electrical potential appliedat the CE column entrance. In the CE/ES apparatus described, theelectric field maintained across the dialysis membrane is in theopposite direction required to supply charge for positive polarityElectrospray ionization. As described by Severs et. al. [9, 10] thesecond solution with electrolyte added is a static solution volumeplaced in a capillary tube surrounding the CE column exit end. Thecapillary tube has open ends to allow release of gas formed in redoxreactions at the CE column exit electrode surface. The secondelectrolyte solution appears to remain in place due to surface tensionof the liquid in the capillary tube. The authors report changing thesecond solution between CE/ES/MS runs, replacing the ammonium acetatesolution with an acetic acid solution, resulting in a shift in chargestate of multiply charged peaks appearing in mass spectrum of myoglobinand carbonic anhydrase. The shifting of the multiply charged profile toincreased charge state peaks would occur with a reduction of pH in theCE solution. How this apparent decrease in pH occurs is not explained bythe authors. The electric field applied across the membrane duringCE/ES/MS with the apparatus described would have driven positivelycharged protons from the CE column solution into the second electrolytesolution effectively decreasing pH in the CE solution. One explanationcould be that a portion of acetic acid in second solution remains in aneutral form and neutral acetic acid molecules may have transferredthrough the dialysis membrane into the CE solution driven by aconcentration gradient during CE/ES/MS operation.

As described in the prior art, it may be desireable in some analyticalapplications to cause redox reactions with sample substances in solutionprior to Electrospray MS analysis. However, for many applications it ispreferable to minimize any changes to the analyte species in solutionprior to Electrospraying to achieve minimum distortion of informationregarding a solution composition in ES/MS analysis. In many applicationsincluding quantitative analysis, the study of peptides and proteins,high throughput screening, drug discovery, drug metabolite studies andbiomarker detection it is preferred to have minimum modification of theanalyte population during ES/MS analysis. The Electrospray probeapparatus configured according to the invention allows control of theElectrospray current using only the Electrospray electric field whilepreventing redox reactions from occurring on conductive surfaces in thefirst or sample solution flow path during Electrospray ionization. Oneembodiment of the invention provides control of the total Electrospraycurrent and sample solution pH while preventing redox reactions fromoccurring on conductive surfaces in the sample solution flow path. Thiscontrol of the Electrospray process allows optimization of ES/MS orES/MS^(n) analysis and expansion of ES/MS^(n) or liquid chromatographyElectrospray mass MS (LC/ES/MS^(n)) analytical capability while insuringminimum modification of the analytes in the sample solution due to redoxreactions prior to Electrospraying. The introduction of specific neutralor charged species into the sample solution through semipermeablemembranes during Electrospray ionization can be selected and controlledto maximize ion signal for different classes of analyte compounds in thesample solution. The invention allows conducting of conductivity or pHscans during Electrospraying to maximize ion signal or to studyprocesses occurring in solution such as protein folding as a function ofpH. Preventing redox reactions from occurring on conductive surfaces inthe first or sample solution flow path minimizes the carryover ofcontamination species that deplate from the conductive surfaces when theElectrospray polarity is changed. The contamination ions occurring inmass spectra when polarity is changed can reduce sample signal due tocharge competition and cause interference peaks in the acquired massspectrum. The charged droplet sprayer configured according to theinvention reduces the time and solvent consumption required to flushsample solution flow paths, providing increased analytical throughput atlower cost per analysis.

The electrical circuit equivalence of conventional Electrosprayionization charged droplet formation and neutralization processes havebeen described by Kebarle, P., and Tang, L., Anal. Chem. 1993, 65,972A-985A [11] and Jackson, G. S., and Enke, C. G., Anal. Chem. 1999,71, 3777-3784 [12]. The total electrical current generated in unassistedor pneumatic nebulization assisted Electrospray is established byelectrolytic processes occurring in solution. For a given voltagedifferential applied between the Electrospray tip and counter electrodesand for a given liquid flow rate, the total Electrospray currentproduced through the formation of charged liquid droplets is a strongfunction of the resistance, or inversely the conductivity, of thesolution being Electrosprayed. The invention allows changing of theeffective solution conductivity during Electrospraying by changing ofthe conductivity of a second solution flow separated from the samplesolution flow by a semipermeable membrane. Charged species exchangedacross the membrane between the first or sample solution and the secondsolution, effectively changing the conductivity of the sample solution,are driven across the membrane by the applied Electrospray electricfield. Selected neutral species may also traverse the membrane driven bya concentration gradient between the first and second solutions that mayalso change the first solution conductivity. The controlled exchange ofproton charged species across the membrane changes the first solutionconductivity and pH. The invention allows the addition of protons orcations to the sample solution during positive polarity Electrosprayionization without the addition of the counter ion as is the case whenacids or salts are added directly to the sample solution. The converseis true for negative polarity Electrospray ionization.

The total Electrospray current can be changed with precise and stablecontrol during Electrospray ionization with no change to the chargeddroplet sprayer geometry or the applied Electrospray voltage. For agiven solution flow rate, as the total Electrospray current increases,the size of the charged droplets produced decreases. Higher totalElectrospray currents with smaller droplet size distributions allowsfaster drying of charged droplets and the reduction of aerosols producedfrom evaporating droplets with insufficient charge available to ionizenon volatile components within the droplet. In unassisted Electrospraycharged droplet production, each initial charged droplet breaks off withapproximately half the Rayleigh limit of charge per droplet. For a givenliquid flow rate, as the total ES current increases due to increasingsolution conductivity, the total number of droplets produced mustincrease to carry the additional charge limited by the Rayleigh limit ofcharge per droplet. As the number of charged droplets produced per timeincreases, the charge to solution volume ratio increases. The sametrends apply with pneumatic nebulization assisted Electrosprayionization charged droplet formation. Increasing the total chargeavailable will increase analyte ES/MS^(n) signal to the point wheresufficient charge is available to ionize all analytic molecules.Increasing the total ES current beyond the equivalent analyteconcentration may cause a decrease in ES/MS^(n) signal. The chargeddroplet sprayer configured according to the invention allows rapidadjustment of total ES current during Electrospray ionization tomaximize analyte signal in ES/MS^(n) analysis.

Embodiments of the invention include charged droplet sprayers configuredsuch that no redox reactions occur on conductive surfaces in the firstor sample solution flow path during charged droplet formation inElectrospray ionization. In one embodiment of the invention, chargedspecies are added to or removed from the first or sample solutionthrough semipermeable, dielectric membranes separating the firstsolution from a second solution or gas flow. In this embodiment, thetotal charged droplet spray current produced from the charged dropletspraying process can be adjusted by modifying the second solution or gasphase composition, electric field strength across the membrane,electrode composition and geometry, membrane composition and geometry,the electric field at the spray tip, the number of spray tips, solutionflow rate and other variables independent of the initial first or samplesolution composition as will become apparent in the description of theinvention. Through adjustment of such variables using the chargeddroplet sprayer configured according to the invention, charged dropletspraying can be optimized for a given application. For example, theamplitude of multiply charged peaks of proteins in a mass spectrumacquired by Electrospraying from an aqueous solution can be increased byadding protons through a fluorethylene polymer (Nafion™) dielectricmembrane during Electrospraying using one embodiment of the invention.Alternative embodiments of the invention provide for charge separationand the addition or removal of net charge from the first or samplesolution with all or a portion of the total charge droplet spray currentgenerated through redox reactions occurring on conductive electrodesseparate from the first solution flow path. Embodiments of the inventionallow adjustment and optimization of charged droplet spraying for agiven sample solution composition.

SUMMARY OF THE INVENTION

The invention comprises embodiments of charged droplet sprayers thatprovide increased performance and the ability to optimize chargeddroplet spray performance over a range of operating conditions andapplications. In one embodiment of the invention, the charged dropletsprayer comprises a first and a second solution flow channel separatedby a single or layered semipermeable dielectric membrane. Selectedcharged species are transferred into or removed from the first solutionthrough the membrane creating a net increase in one polarity charge inthe first solution flow during charged droplet spraying. The firstsolution, with an increase in one charge polarity, forms a spray ofcharged droplets at one or more first solution flow channel exit tips.The transfer of charged species through the membrane and the productionof the charged droplets from the first solution flow channel exit tipare driven by the Electrospray electric field maintained at the firstsolution flow channel exit tip. The membrane and the first and secondsolutions form electrically resistive conduits between the Electrosprayelectric field present at the first solution flow channel exit tip andan electrode surface positioned in the second solution flow channel incontact with the second solution. The Electrospray electric fieldmaintained at the first solution flow channel exit tip is established bythe relative electrical potentials applied to counter electrodes spacedfrom the exit tip and the electrical potential applied to the electrodein contact with the second solution in the second solution flow channel.The charged species transferred into or removed from the first solutionflow through the membrane is determined by selection of the membranecomposition, composition of the second solution electrode, compositionand flow rates of the first and second solutions and the polarity of theelectric field across the membrane. Positive and negative polaritycharged droplet spray current can be optimized for a given applicationby adjusting the variables of solution chemistries and flow rates,relative electrical potentials applied to electrodes and by theselection of membrane materials. Total Electrospray current can bechanged during Electrospray ionization by changing the second solutioncomposition and/or first solution flow rate.

Protons can be transferred from the second solution into aqueous samplesolutions to increase solution charge and decrease solution pH duringpositive polarity charged droplet spraying without adding acid speciesdirectly to the first solution. Redox reactions occur at conductiveelectrode surfaces positioned in one or more second solution flowchannels driven by the Electrospray electric field. The same electricfield drives the charged species across the membrane between the firstand second solutions. Deposition of anions on first solution flowchannel conductive surfaces is minimized or eliminated during positiveion polarity Electrospray. This avoids deplating of anions fromconductive surfaces in the sample solution flow path when theElectrospray ion polarity is reversed. The interference anions producedby deplating from conductive surfaces in negative polarity ES can resultin charge suppression of analyte species and the occurrence of unwantedcontamination peaks in acquired mass spectra. The converse holds whenswitching from negative to positive polarity Electrospray ionization. Inanalytical applications requiring upstream sample separation techniquessuch as in LC/ES/MS^(n) analysis, conductive surfaces cannot be entirelyeliminated in upstream sample solution flow paths due to the presence ofupstream LC columns, valves, fittings and pumps. In such cases, thevoltage applied to the electrode in contact with the second solution canbe adjusted to minimize or eliminate the occurrence of redox reactionson upstream conductive surfaces in the sample solution flow channel.Embodiments of the invention enable the generation of charged dropletsprays in which the total Electrospray or charged droplet spray currentproduced is greater than the electrical current generated due toreduction or oxidation reactions occurring on conductive surfaces in thefirst solution flow channel. In charged droplet sprayers configuredaccording to the invention, redox reactions supplying electrical currentto the charged droplet formation process in Electrospray occur onelectrode surfaces configured external to the first solution flowchannel.

In alternative embodiments of the invention, charged droplet sprayerscan be configured with the first solution separated from multiple secondsolutions by individual membranes comprised of similar or differentmaterials. Different charged species can be individually orsimultaneously added to and/or removed from the first solution duringcharged droplet spraying using multiple membrane embodiments. The firstsolution flow channel may be configured to terminate with single ormultiple exit tips. Generating multiple charged droplet sprays frommultiple exit tips allows an increase in the total charged droplet spraycurrent produced for a given first solution composition and allowsoptimization of the overall charged droplet spray geometry for specificapplications. Charged droplet sprays with single or multiple exit tipscan be formed using unassisted Electrospray or pneumatic nebulization ofsolution in the presence of an electric field, alternatively describedas Electrospray with pneumatic nebulization assist.

An alternative embodiment of the invention comprises first and secondsolution flow channels separated by a semipermeable dielectric membraneconfigured with an insulated porous electrode positioned adjacent to thefirst solution side of the membrane or configured between membranelayers. The electric field formed between the insulated porous membraneand the electrode configured to be in contact with the second solutionin the second solution flow channel can be adjusted to increase ordecrease charge species transfer through the membrane. The addition ofthe insulated porous membrane allows additional control of chargedspecies transfer into or out of the first solution without the need toadjust solution chemistry in the first or second solutions duringcharged droplet spraying. The charged droplet sprayer can be configuredwith multiple second solution flow channels separated from the firstsolution by separate membranes. Conversely, the charged droplet sprayercan be configured with multiple first solution channels separated from asecond solution flow channel by separate membranes. The multiple firstsolution flow channel configuration allows the simultaneous spraying ofpositive and negative polarity charged droplets from two sprayer exittips using the same or different first solutions. Alternately, chargeddroplet sprays of the same polarity may be generated from the twosprayer tips from different first solutions.

An alternative embodiment of the invention comprises a single firstsolution flow channel configured with two exit tips with only dielectricsurfaces or no connected conductive surfaces present in the firstsolution flow channel where reduction or oxidation (redox) reactions canoccur. Opposite polarity charged droplets of the first solution aresprayed simultaneously from the two exit tips toward counter electrodeshaving different electrical potentials applied. Such dual output, dualpolarity charged droplet sprayer may be combined with the membraneseparated first and second solution flow channel sprayer embodimentdescribed above to allow addition or removal of one charged species tothe first solution through the membrane and bifurcation of chargespecies in the first solution flow path during charged droplet spraying.Using such combined charged droplet sprayer configuration, the totalcharged droplet spray current of opposite polarity may not be equal fromboth tips. Such current balance can be adjusted by selecting theappropriate relative electrical potentials applied to electrodes. One ofthe two exit tips may be positioned sufficiently close to a counterelectrode such that solution leaving the exit tip forms an electricalcontact with the counter electrode without spraying. Using thisembodiment, separation of charge in the first solution can be achievedduring charged droplet spraying while avoiding redox reactions onsurfaces in the first solution flow channel and without the need tooptimize two charged droplet sprays simultaneously. Finer control of theremaining single charged droplet spray can be achieved by adjustingsolution chemistry or applied voltages using such dual outlet embodimentemploying solution contact to the counter electrode. Alternatively, sucha second solution channel may be terminated with an end electrodeallowing electrical contact with the first sample solution removed fromthe first solution flow path while preventing loss of sample solutionflow to the electrode.

In an alternative embodiment of the invention, the first sample solutioncomposition may be modified during charged droplet spraying through aliquid junction configured between a first and second solution flowchannel of a dual output opposite polarity charged droplet sprayerembodiment. The geometry of the liquid junction between both solutionscan be configured to maximize or minimize contact between the twosolutions while allowing exchange of charged species. The dual flowchannel charged droplet sprayer embodiment may be configured andoperated to prevent flow of the first solution into the second solutionflow channel, allow flow of the first solution into the second solutionor vice versa, during charged droplet spraying. As described above, tosimplify optimization and control of charged droplet spraying from oneexit tip, the second flow channel exit tip can be positionedsufficiently close to a counter electrode such that the liquid leavingthe exit tip forms an electrical contact with the counter electrode.Charged droplet spray can be generated from the first solution flowchannel exit tip using Electrospray or Electrospray with pneumaticnebulization assist. Embodiments of the invention may be combined toallow more flexibility and range in controlling the charged dropletspray process. Increased control of the charged droplet formationprocess and the sample solution composition during Electrosprayionization allows enhancement and optimization of ES/MS^(n) andLC/ES/MS^(n) performance for given applications. Charged dropletspraying can be conducted using the embodiments of the invention orusing combinations of embodiments of charged droplet sprayer devicesconfigured according to the invention whereby the charged droplet spraycurrent produced is greater than the electrical current generated due toredox reactions occurring on conductive surfaces in the first solutionflow channel.

BRIEF DESCRIPTION OF THE INVENTION

FIG. 1 is a cross section view of single exit tip charged dropletsprayer assembly with pneumatic nebulization comprising first and secondsolution flow channels separated by a membrane.

FIG. 2 is a cross section view of a two flow channel membrane assemblyconnected to a separate pneumatic nebulization charged droplet sprayer.

FIG. 3 is a diagram of an electrochemical reaction in second solutionflow channel with proton exchange across the membrane of the chargeddroplet sprayer embodiment shown in FIGS. 1 and 2.

FIG. 4A is a mass spectrum of hexatyrosine sprayed from a 100% aqueoussolution using the charged droplet sprayer embodiment shown in FIG. 1.

FIG. 4B is an extracted ion chromatogram of hexatyrosine in a 100%aqueous solution sprayed from the charged droplet sprayer embodimentshown in FIG. 2.

FIG. 5A is a mass spectrum of a 3.5 μM solution of Progesterone in a 1:1acetontrile:water, 0.1% glacial acetic acid sprayed using a conventionalElectrospray probe.

FIG. 5B is a mass spectrum of a 3.5 μM sample solution of Progesteronein a 1:1 acetontrile:water with a 0.67% glacial acid in water secondsolution sprayed from the charged droplet sprayer embodiment shown inFIG. 2.

FIG. 6A is a mass spectrum of a 180 μM solution of Anthracene inAcetontrile with 0.5% HCL sprayed using a conventional Electrosprayprobe.

FIG. 6B is a mass spectrum of a 180 μM sample solution of Progesteronein acetontrile with a 0.83% HCL in water second solution sprayed fromthe charged droplet sprayer embodiment shown in FIG. 2.

FIG. 7 is an extracted ion chromatogram for Hexatyrosine and Anthracenerunning pH scans in the second solution while spraying from the chargeddroplet sprayer embodiment shown in FIG. 2 compared to conventionalElectrospraying with increasing acid concentration in the samplesolution.

FIG. 8A is a mass spectrum of a 1.2 μM solution of Myoglobin in waterwith a 100% aqueous second solution sprayed from the charged dropletsprayer embodiment shown in FIG. 2.

FIG. 8B is a mass spectrum of a 1.2 μM solution of Myoglobin in waterwith a 0.83% HCL in water second solution sprayed from the chargeddroplet sprayer embodiment shown in FIG. 2.

FIG. 8C is a mass spectrum of a 1.2 μM solution of Myoglobin in waterwith a 2.5% HCL in water second solution sprayed from the chargeddroplet sprayer embodiment shown in FIG. 2.

FIG. 8D is a mass spectrum of a 1.2 μM solution of Myoglobin in watersolution with a 4.17% HCL in water second solution sprayed from thecharged droplet sprayer embodiment shown in FIG. 2.

FIG. 9 is a diagram of a hydrogen gas oxidation reaction in the secondflow channel with proton transport across the membrane in the chargeddroplet sprayer embodiment shown in FIGS. 1 and 2.

FIG. 10A is a diagram of a charged droplet spray comprising a singlestable Electrospray plume.

FIG. 10B is a diagram of a charged droplet spray comprising two stableElectrospray plumes.

FIG. 11 is a cross section view of a multiple exit tip charged dropletsprayer assembly comprising first and second solution flow channelsseparated by a membrane.

FIG. 12 is a cross section view of a multiple exit tip charged dropletsprayer assembly with pneumatic nebulization comprising first and secondsolution flow channels separated by a membrane.

FIG. 13 is a cross section view of a charged droplet sprayer assemblywith pneumatic nebulization comprising one first solution flow channelseparated from two second solution flow channels by two membranes.

FIG. 14 is a cross section view of a charged droplet sprayer assemblywith pneumatic nebulization comprising one second solution flow channelseparated from two sample solution flow channels by two membranes.

FIG. 15 is a cross section diagram of a charged droplet sprayer assemblywith pneumatic nebulization comprising a first and second solution flowchannel separated by a membrane and a porous insulated electrode.

FIG. 16 is a cross section view of the first and second solution flowchannels separated by a dielectric membrane and porous insulatedelectrode.

FIG. 17 is cross section view of a charged droplet sprayer withpneumatic nebulization comprising one first solution flow channelseparated from two second solution flow channels by two dielectricmembranes and two porous insulated electrodes.

FIG. 18 is a cross section diagram of a charged droplet sprayer withpneumatic nebulization configured to simultaneously spray positive andnegative polarity charged droplets from two sprayer tips.

FIGS. 19A, B and C are cross section diagram views of a charged dropletsprayer with pneumatic nebulization comprising two outlet channels withone outlet forming a solution contact with an electrode.

FIG. 20 is a cross section diagram of a charged droplet sprayercomprising two solution inlets into a common flow channel and twosprayer outlets producing charged droplet sprays of opposite polarity.

FIG. 21 is a cross section diagram of a charged droplet sprayercomprising separate first and second solution inputs and outputs with afluid connection between the first and second solution flow channels.

DESCRIPTION OF THE INVENTION

Charged liquid droplets can be formed in charged droplet spray devicesusing unassisted Electrospray or pneumatic nebulization in the presenceof an electric field. Charged droplet production in unassistedElectrospray requires the formation of a stable Taylor cone jet from thesample solution exiting a channel or tube in the presence of an electricfield. The charged droplets form by separating from a liquid filamentprotruding from the tip of the Taylor cone. If a sample solution hashigh surface tension, it may not be possible to form a stable Taylorcone at atmospheric pressure where electrical potentials applied have anupper limit due to gas phase break down. If the conductivity of a samplesolution is too high, the filament projecting from the Taylor cone maynot separate into uniform charged droplets due to damping of the fluidcolumn instability by charge movement within the solution. Stable Taylorcones are more difficult to sustain at higher liquid flow rates. Bothultrasonic and pneumatic nebulization charged droplet sprayer deviceshave been reported and both nebulization techniques can be applied tothe embodiments of the invention described below. Pneumatic nebulizationsprayer devices are most widely used for the generation of chargedliquid droplets from sample solutions. Pneumatic nebulization chargeddroplet sprays form from a channel or tube tip in the presence of anelectric field by pneumatically shearing the solution as it exits thetube. The gas shearing force acting on the exiting liquid stream issufficient to create charged droplet sprays even for higher surfacetension and higher conductivity solutions and for higher liquid flowrate operating conditions. The Taylor cone and liquid filament structureformed in Electrospray to generate charged liquid droplets does notexist in pneumatically nebulized charged droplet sprays. Consequently,charged droplet production using Electrospray (unassisted Electrospray)or pneumatic nebulization in the presence of an electric field(Electrospray with pneumatic nebulization assist) are described inrelation the invention as two distinct processes. Both processes achievethe production of charged droplets but each has a different performanceresponse with respect to the invention and each generate differentcharged droplet size distributions.

Using unassisted Electrospray or pneumatic nebulization in the presenceof an electric field to form charge droplet sprays, the charged dropletcurrent generated is a function of the conductivity of the solution,location and material of the conductive surface in the fluid flow path,the location of the reduction-oxidation (redox) reaction in the fluidpath, the liquid flow rate, the externally applied electric fieldstrength, the solution composition and the flow channel and flow channelexit tip material and geometry. The charged species or ions formed fromevaporating liquid droplets are a function of the sample solutioncomposition, the flow channel conductive material, the totalElectrospray current and the droplet drying conditions. The inventionprovides control of the transfer of known and selected charged speciesinto or out of the sample solution flow channel and provides control ofthe total charge produced by through the charged droplet spray processwith the same initial sample solution composition. In one embodiment ofthe invention, the reduction-oxidation reactions required for chargeseparation during charged droplet production occurs in a second gas orsolution flow channel separated from the first or sample solution flowchannel by a semipermeable dielectric membrane. The total number ofcharge species transferred through the membrane per time period can beadjusted by modifying the composition of solution or gas flowing throughthe second channel and the voltage applied to the electrode configuredin the second solution flow channel. In another embodiment of theinvention, the total charge generated by the charged droplet spray ismodified by changing the electrical potentials applied to electrodespositioned within the first and second flow channels. In a thirdembodiment of the invention, charge separation is achieved in thesolution flow channel by splitting the flow into a positive and negativeelectrically biased channels. The ability to control the transfer ofcharged species, total charged droplet spray current and the location ofthe reduction-oxidation reactions, independent of the first solutioncomposition and flow path allows the optimization of Electrospray andnebulization assisted Electrospray charged droplet spray performance inatmospheric pressure ion sources interfaced to mass spectrometers and inother applications. The charged droplet sprayer configured according tothe invention allows modification to the Electrospray ionization processusing direct user or computer program control.

FIG. 1 is a cross section diagram of one embodiment of the inventionwhere flow channel 1 and flow channel 2, are separated by membrane 3 incharged droplet sprayer assembly 4. Charged droplet sprayer assembly 4comprises conductive element or electrode 5 in contact with the liquidor gas flow through channel 2 and dielectric body 8 in contact with theliquid flow through channel 1. Conductive element 5 is mounted insprayer body 6 comprising dielectric or conductive material. Flowchannel 2 is configured for gas or liquid flow through channel 2 ofsprayer assembly 4. Dielectric or electrically insulating body 8 isconfigured with channel 1 to allow flow of sample solution along thesurface of membrane 3 with minimum dead volume. Channel 1 may beconfigured with greater width than depth to maximize solution contactarea with membrane 3 while minimizing flow channel 1 dead volume.Minimum dead volume reduces sample carryover or band broadening when thecharged droplet sprayer is used in ion sources interfacing liquidchromatography and mass spectrometry (LC/ES/MS) instruments. Membrane 3separates conductive element 5 from sample solution 1A flow and servesas a seal surrounding flow channels 1 and 2 clamped between sprayer bodycomponents 6 and 8. Electrode 7 is electrically isolated from liquidflow channel 1 by dielectric body 8.

A first or sample solution 1A enters channel 1 through entrance tube 10,passes through flow channel 1 and tube 9 flow channel 11 exiting fromexit tip 12 as a charged droplet spray. Sample solution 1A flow throughchannel 1 is delivered and controlled through upstream fluid delivery orseparation system 13. Sample solution delivery system 13 may include butis not limited to a liquid chromatography separation system, syringepump, solution reservoir or capillary electrophoresis system. A secondgas or solution 2A enters channel 2 through tube 15, passes throughchannel 2 and exits through tube 16. Conversely, gas or solution 2A mayenter channel 2 through tube 16 and exit through tube 15. Gas orsolution 2A can be supplied from a gas or fluid delivery system 34through connecting channel 14. As will be described in more detailbelow, gas or fluid delivery system 34 can be operated to change thesecond gas or solution composition during Electrospray ionization.Stepped or gradient second gas or liquid composition profiles can be runduring Electrospray ionization under user or program control. Gas orfluid delivery system 34 can change second gas or solution 2Acomposition based on user input, time periods, software programmedprofiles or in response to data dependent events.

When pneumatic nebulization is employed for charged droplet formation,nebulization gas 18 is supplied through entrance tube 19, passingthrough annulus 17 and exits as high velocity gas flow 20 surroundingexit tip 12. Nebulizer annulus tube 39 may be configured as anelectrically conductive or as a dielectric material in the embodimentshown. Electrical potentials applied to ring electrode 21, endplateelectrode 22 and capillary entrance electrode 23 form an electric fieldat exit tip 12 of flow channel 1 during charged droplet spraying.Electrical voltages are applied to electrodes 5, 7, 21, 22 and 23through power supplies 27, 28, 29, 30 and 31 respectively through useror software control. In some operating modes, an electrical potentialcan be applied to an upstream conductive element in the first solutionflow channel such as entrance tube 10, configured as a conductor,through power supply 32 when switch 33 is closed. Typically upstreamconductive surfaces such as connecting tubing, fittings and/or LC pumpsare connected to ground potential. When it is preferable to have noredox reactions occurring on conductive surfaces in the first solutionflow channel 1, conductive elements in the upstream liquid deliverysystem flow channel can be electrically floated or disconnected from anelectrical reference by opening switch 33. Alternatively, the firstsolution flow pathway 1, including tube 10 can be configured withdielectric material. In such an embodiment, the first solution flowchannel is electrically isolated or floating. An electrically isolatedfluid delivery system may comprise a dielectric or electrically floatedsyringe. When it is not practical to electrically float conductivesurfaces in the upstream first solution flow channels, the voltageapplied to electrode 5 can be set to minimize or prevent redox reactionsfrom occurring on upstream first solution flow channel conductivesurfaces during Electrospray ionization.

Charged droplets are produced using charged droplet sprayer 4 fromsolution 1A flowing through channel 1 and spraying from exit tip 12 byapplying an electrical potential difference between electrode 5 andexternal electrodes 21, 22 and 23. Electrode 7 with voltage supply 28may be included or removed from the charged droplet sprayer depending onthe required operating mode. Electrode 21 may also be removed providedthat appropriate electrical potentials are applied to the remainingcounter electrodes 22 and 23 during charged droplet spraying. Tube 9 maycomprise a dielectric material such as fused silica or PEEK(polyetheretherketone) or conductive material such as stainless orplatinum. In the embodiment of the invention shown in FIG. 1, when tube9 comprises a conductive material, it is electrically isolated indielectric body 8 to prevent any redox reactions from occurring on thesurface of channel 11 of tube 9 during charged droplet formation.Depending on the presence of connected conductive elements in flowchannel 1, charged species transferred through membrane 3 in chargeddroplet sprayer 4 provide all or a portion of the charged droplet spraycurrent during Electrospray ionization. Charged and/or neutral speciespassing through membrane 3 modify the composition of sample solution 1Aduring Electrospray ionization in the portion of flow channel 1 and 11from membrane 3 to exit tip 12. During positive polarity charged dropletspraying, positive electrical potential is applied to electrode 5relative to the electrical potentials applied to counter electrodes 21,22 and 23. The electric field formed at exit tip 12 drives the movementof charged species in solution 1A along channel 11, in tube 9, alongchannel 1, through dielectric membrane 3 and across channel 2 toelectrode 5.

In positive polarity Electrospray ionization operating mode with theappropriate gas or solution 2A flowing through channel 2 and theappropriate electrode material 5, such as graphite, protons (H+) areformed by an oxidation reaction occurring at the surface of cathode 5.Electrons flow from the surface of electrode 5 to power supply 27 aselectric current. The protons formed move through semipermeabledielectric membrane 3 from channel 2 into channel 1, driven by theelectric field, forming a net positively charged solution 1A in channel1. Positively charged solution 1A passes through channel 1 and channel11 and sprays from exit tip 12 forming positive polarity chargeddroplets. The charged species produced from evaporating positivepolarity charged droplets that impinge on negative potential counterelectrodes 22 and 23, neutralize by accepting electrons from powersupplies 30 and 31, completing the electrical circuit for that portionof charge. A portion of the charged species formed from the evaporatingcharged droplets enter orifice 24 into vacuum and are mass analyzed bymass analyzer 35. The positive polarity charged species entering vacuumare neutralized by impinging on conductive surfaces or the massspectrometer detector, completing the electrical circuit for thatportion of charge produced. The positive polarity charged droplet sprayremoves positive charge from flow channel 1 and 2 effectively completingthe electrical circuit with power supply 27. Typically, for an exit tip12 to counter electrode 22 spacing of 1 to 2 centimeters, a 3,000 to6,000 volt differential will be maintained between electrical potentialsapplied to electrode 5 and counter electrode 22 when electrode 21 is notpresent. Lower voltages are typically applied when smaller spacings areconfigured between counter electrode 22 and exit tip 12 to maintain asufficiently high electric field at exit tip 12 to produce chargeddroplet spray 38 while avoiding gas phase breakdown, the formation ofcorona discharge or unstable Taylor cones. One or more Taylor cones mayform at tip 12, without nebulization gas flow, producing charged liquiddroplets through the Electrospray process. Alternatively, nebulizationgas flow 20 can be applied to form charged liquid droplets through gasto liquid shear forces at exit tip 12 without the formation of a Taylorcone. Negative charged liquid droplets are formed by reversing thepolarity of the relative potentials described above. In both positiveand negative charged droplet production, electrode 5 may be maintainedat or near ground potential with kilovolt potentials applied to counterelectrodes 21, 22 and 23. Conversely, kilovolt potentials may be appliedto electrode 5 with electrodes 21, 22 and 23 maintained closer to groundelectrical potential.

When charged droplet sprayer 4 is configured in an atmospheric pressureion source for mass spectrometry, the charged liquid droplets formed inspray 38 are directed toward counter electrodes 22 and 23 by the appliedelectric field against a heated counter current drying gas 25 flowingthrough opening 37 in endplate electrode 22. Heated counter currentdrying gas 25 aids in drying the charged liquid droplets formed in spray38. As the charged liquid droplets evaporate, ions are formed and aportion of the ions are swept through orifice 24 into vacuum where theyare mass to charge analyzed using mass to charge analyzer 35. Chargeddroplet sprayer 4 and alternative embodiments as described in thefollowing sections may be used in other applications where chargedliquid droplets or ions created from evaporating charge liquid dropletsare required. Such applications may include spray painting or ionimplantation on surfaces. The charged droplet sprayer may be configuredwith ion sources that employ gas phase charge exchange or chargeimpingement on surfaces. For example, the charged droplet sprayer 4 maybe configured to direct charged droplets counter flow to a vaporizedsample solution flow in an Atmospheric Pressure Chemical Ionization(APCI) source to provide a field of charged ions for gas phase chargeexchange with vaporized gas phase sample molecules. In such anembodiment, charged droplet sprayer 4 eliminates the need for a coronadischarge needle to create gas phase ions as configured in aconventional APCI source. In an alternative application, ions formedfrom the charged droplet sprayer can be directed to impinge on a sampletarget. Formation of sample ions from such sample target surfaces can begenerated by collision of charge droplet sprayer generated ions with thesurface, rapid reversal of the electric field at the surface aftercharging and with impingement of a laser pulse after charging of thesurface as is described in pending U.S. Provisional Patent ApplicationSer. No. 60/573,666 incorporated herein by reference.

An alternative embodiment to the invention is diagrammed in FIG. 2.Pneumatic nebulizer sprayer assembly 43 is configured separate from twosolution flow channel membrane assembly 40 in charged droplet sprayerassembly 44 shown in FIG. 2. All elements in FIG. 2 that are common tothose elements shown in FIG. 1 retain the same numbers. Sample solution1A flows through solution channel 1, through flow channel 47 in tube 43and exits at tip 12 with pneumatic nebulization gas flow 20. Tube 43comprises an electrically floating conductive material such as stainlesssteel or a dielectric material such as fused silica. Tube 43 can beconnected to two flow channel membrane body component 45 usingconventional means including, but not limited to, a ferrule and nuttubing connection. Sprayer assembly 43 is positioned at an anglerelative to ion source centerline 48 to avoid spraying charged dropletsinto orifice 24 in higher liquid flow rate applications. Membraneassembly 40 body components 45 and 46 comprise flat surfaces that wouldform a flush contact against semipermeable membrane 3 without slottedgaskets 41 and 42. Dielectric slotted gaskets 41 and 42 are positionedbetween body components 45 and 46 respectively and membrane 3 in twoflow channel membrane assembly 40. Gaskets 41 and 42, typicallycomprising a dielectric material, seal flow channels 1 and 2 when bodyelements 45 and 46 are clamped together. The cross sectional area offlow channels 1 and 2 are established by the gasket thickness and widthof the opening or slot in gaskets 41 and 42 respectively. Body element45 comprises a dielectric material and body element 46 comprises adielectric or conductive material. When body element 46 is comprises aconductive material it is configured as electrically insulated fromsurrounding elements but in electrical contact with electrode 5. Theseparation of two flow channel membrane assembly 40 from pneumaticnebulization sprayer assembly 43 allows two flow channel membraneassembly 40 to be interfaced to commercially available pneumaticnebulization or unassisted Electrospray inlet probes in Electrospraymass spectrometer instruments.

The composition of sample solution 1A can be altered in flow channel 1by the flow of charged and neutral species through membrane 3 duringElectrospray ionization. Charged species are formed in flow channel 2from electrochemical reactions occurring between gas or solution 2Apresent in flow channel 2 with the surface of electrode 5 in thepresence of an electric field. Charged species formed from electrontransfer between solution or gas 2A and electrode 5 are transferredthrough membrane 3 driven by the same electric field. FIG. 3 is a crosssection diagram of flow channels I and 2, membrane 3, electrode 5 anddielectric body 8 or 45 in charge droplet sprayers 4 and 44 as shown inFIGS. 1 and 2. FIG. 3 is a diagram of one example of the formation ofpositive charged species and exchange of charged species from solution2A into solution 1A across membrane 3 in positive polarity Electrosprayionization. Sample solution 1A enters flow channel 1 at end 50comprising molecular species C₁, C₂ and C₃ that ultimately formprotonated ions from sprayed charged droplets provided each species hassufficient proton affinity. Second solution 2A flows through flowchannel 2 entering at end 52 and exiting at end 53 comprising molecularspecies M₁, M₂ and M₃. Charged species move across channel 2 fromelectrode 5 through membrane 3 driven by application of an electricfield at Electrospray tip 12 as described above. Charged species passingthrough flow channel 1 through exit end 51 during Electrosprayionization form a portion of the electric circuit terminating in theexample shown at the positive polarity end with power supply 27. In theexample shown in FIG. 2, the M₁ species is water (H₂O) and M₂ and M₃species comprise an appropriate electrolyte, such as hydrochloric acid,acetic acid or a salt to aid in the electrolysis of water at the surfaceof electrode 5. The surface of electrode 5 may comprise silver, silverchloride, carbon, gold, platinum black, platinum, stainless steel orother appropriate material that will maximize electrolysis efficiencybut minimize electrode erosion.

When operating in positive polarity charged droplet spray mode, apositive electrical potential is maintained on electrode 5 using powersupply 27 relative to the potentials applied to counter electrodes 22and 23. Electrolysis of water molecules M₁ occurs at electrode 5 formingH+, oxygen (O₂) and other species as described by Van Berket et. al.[4].H⁺ is driven by the electric field across channel 2 toward membrane 3.Membrane 3 comprises an appropriate material to selectively facilitateproton transfer through the membrane but provides an otherwise inertimpermeable dielectric surface to solution 1A in channel 1 and solution2A in channel 2. In one preferred embodiment of the invention, membrane3 material comprises sulfonated fluoroethylene material,(perfluorsulphonic acid polytetrafluoroethylene (PTFE) copolymer) oneformulation of which is Nafion (® Dupont). Nafion is a fluorethylenepolymer with sulfonated side chains terminating with an ionically bondedsulphonic acid (HSO₃) that forms an SO₃ ² ion at the side chaintermination. The hydrophyllic property of the sulphonic acid groupscauses local hydration of an otherwise hydrophobic material. Othermembrane materials, including but not limited to, cellulose esters andpolysulfone dialysis tubing with different molecular weight cutoffs orcation or anion semipermeable membranes available from Dionexcorporation may be configured as membrane 3. Specific membranes can beused that maximize performance for a given applications. The same ordifferent material membranes can be layered to enhance specific speciespermeability while selectively blocking unwanted species. Individualmembrane materials such as Nafion will pass selected charged speciesdriven by an electric field and selected neutral species driven byconcentration gradients between solutions 1A and 2A on opposite sides ofmembrane 3. Such ion and neutral selectivity can be exploited to enhanceperformance in Electrospray MS analysis. The performance of a Nafionmembrane during Electrospray ionization is described below as oneexample of a type of membrane material that can be used in theembodiments of the invention shown in FIGS. 1 and 2.

Driven by the electric field across membrane 3, H⁺ ions are able to passthrough the Nafion membrane moving along the hydrated sulfonated sidechain groups due to the relatively weak attraction of the H⁺ to thehydrated SO₃ ² ion. Nafion is commonly employed in fuel cell technologyto selectively transport H⁺ ions from one flow chamber to another. Theuse of Nafion provides a chemically inert surface to solutions 1A and 2Aflowing through channels 1 and 2 respectively, while allowing thetransfer of protons across the membrane with minimum transport ofunwanted chemical species into solution 1A. Protons move throughmembrane 3 driven by the electric field adding protons into solution 1Aflowing through channel 1. This H⁺ charge transfer forms a net increasein a positive polarity charge in solution 1A flowing through channel 1,without the addition of anion species, supplying positive polaritycharge during the formation of positively charged droplets spraying fromexit tip 12. Electrical potential can be applied to electrode 7 topromote or inhibit charge exchange across membrane 3, however, thepotential applied to electrode 7 exerts less influence on the chargetransfer process than the relative electrical potentials applied betweenelectrode 5 and counter electrodes 21, 22 and 23. 100% aqueous solution,without acid, can be used as solution 2A flowing through channel 2minimizing conductance and reducing the total charged droplet currentproduced from solution 1A. Proton current transferred across membrane 3can be increased for the same relative electrical potentials applied toelectrode 5 and counter electrodes 21, 22 and 23 by increasing theconcentration of acid in solution 2A.

The total current produced from solution 1A sprayed from exit tip 1 canbe increased by increasing the concentration of electrolyte in solution2A flowing through channel 2. The increase in positive polarity spraycurrent produced by increasing the concentration of acid in solution 2Ais similar to that achieved by increasing the concentration of acid insolution 1A in conventional Electrospray ionization. In the embodimentof the invention shown in FIGS. 1 and 2, charged droplet sprayers 4 and44 are configured having no connected conductive elements in the firstsolution 1A flow channel 1. Electrode-solution electrochemical reactionsoccur on surfaces external to the first solution 1A flow path duringcharged droplet spraying. The total charged droplet spray currentproduced from charge droplet sprayers 4 or 44 can be varied by changingthe pH or concentration of acid of solution 2A in channel 2. Theconcentration of acid can be changed as a step function or gradientduring Electrsprah operation using fluid delivery system 34. Forexample, a gradient LC or dual syringe pump can be used for fluiddelivery into channel 2. If the solution in the first syringe is waterand the solution in the second syringe contains water with hydrochloricacid, then the ratios of the two solutions can be controlled by the LCgradient or dual syringe pump prior to delivery to channel 2 of chargeddroplet sprayer 4 or 44. Alternatively, fluid delivery system 34 can beone half of an electrolysis cell comprising a Nafion or otherappropriate semipermeable membrane. The voltage applied acrosselectrodes in the electrolysis cell will determine the concentration ofprotons delivered to solution 2A. Software controlled fluid deliverysystem 34 can be can be programmed to generate specific charged dropletspray currents from Electrospray tip 12 by controlling the rate ofcharge species transfer into or out of solution 1A through membrane 3.Charged droplet spray currents can be controlled in this manner withoutchanging exit tip 12 to counter electrode 22 and 23 geometries orchanging the relative voltages applied between electrode 5 and counterelectrodes 21, 22 and 23. Slow or rapid pH or conductivity scans insolution 1A can be conducted by stepping or ramping the pH in solution2A during Electrospray ionization.

FIG. 4A is a mass spectrum of hexatyrosine Electrosprayed from a 100%aqueous solution 1A using charged droplet sprayer 44 with a 100% aqueoussolution 2A flowing through channel 2 at a flowrate of 3 ul/min. Tube 43in charged droplet sprayer 44 comprised a fused silica tube with nopneumatic nebulization used while acquiring spectra 57. The amplitude ofhexatyrosine peak 58 was stable during acquisition as shown in extractedion chromatogram 59 plotted in FIG. 4B. Solution 1A was not in contactwith conductive elements during charged droplet spraying so allelectrochemical reactions occurred on conductive surfaces external tothe first solution flow path 1A. Acquisition of mass spectrum 57 with MSsignal stability comparable to that shown in FIG. 4 when Electrosprayinga 100% aqueous solution without pneumatic nebulization assist is moredifficult to achieve using conventional Electrospray probes with metaltips. The charged droplet sprayer allows the stable Electrospraying ofsolutions that would be difficult to achieve with standard conductivetip Electrospray probes configured with redox reactions occurring onconductive surfaces in the first sample solution flow path.

FIG. 5 shows a set of ES-MS spectra of progesterone run with a standardconductive tip Electrospray probe and charged droplet sprayer 44 asdiagrammed in FIG. 2, both using pneumatic nebulization assist. FIG. 5Ashows the positive polarity mass to charge spectrum of the protonatedmolecular ion of a 3.5 μM (3.5 pm/μl) solution of progesterone,(M+H)⁺=315.2 m/z, in a 1:1 acetonitrile:water with 0.1% acetic acidElectrosprayed at 10 μl/min. The total Electrospray current was 158nanoamps exceeding the minimum of 56 nanoamps total Electrospray currentrequired to fully protonate 3.5 μM of singly charged sample ionsElectrospraying at a liquid flow rate of 10 μl/min. FIG. 5B shows a massto charge spectrum of the same 3.5 μM solution of progesterone in a 1:1acetonitrile:water solution with no acetic acid added acquired whileElectrospraying from the charged droplet sprayer 44 at a flow rate of 10μl/min. Solution 2A flowing through channel 2 was water with 0.2% aceticacid producing a total Electrospray current of 95 nA. The progestone(M+H)⁺ peak amplitude increased over a factor of two from the maximumsignal achieved using the standard Electrospray probe. Running a pHgradient with hydrochloric acid (HCL) added to solution 2A with 1:1acetonitrile:water solution instead of acetic acid produced a comparable(M+H)⁺ signal at a 1% HCL acid concentration in solution 2A with a totalElectrospray current of 275 nA. All MS spectra shown were acquired usingand Analytica of Branford, Inc. atmospheric pressure ion sourceorthogonal pulsing Time-Of-Flight mass spectrometer.

Improved mass spectrum quality can be achieved using charge dropletsprayers configured according to the invention. Eliminating the need toadd acids, bases, salts or buffer species to the sample solution toincrease solution conductivity or to buffer or modify pH avoids theaddition of contamination species included in such added speciessolutions. FIG. 6A is a positive polarity mass spectrum of the molecularion of non polar Anthracene acquired by Electrospraying a solution 1A of180 μM Anthracene in acetronitrile with 0.05% HCL acid using a standardES probe. The total ES ion current was 590 nA and several contaminatepeaks, possibly added with the HCL acid, are present in the massspectrum. FIG. 6B is a mass spectrum of a 180 μM solution of Anthracenein acetronitrile acquired by Electrospraying at 10 μl/min using thecharged droplet sprayer 44 as diagrammed in FIG. 2 with a 1% HCL acid inwater solution 2A flowing through channel 2. The total ES current duringMS acquisition was 232 nA. The amplitude of the molecular ion peak isconsistent in both spectrum, however, the mass spectrum acquired usingthe charged droplet sprayer 44 shows fewer contamination peaks. PH scansin solution 1A can be conducted during Electrospray ionization usingcharged droplet sprayers 4 or 44, configured according to the invention.Curve 60 of graph 63 in FIG. 7 shows a pH scan conducted for Anthraceneusing the charged droplet sprayer 44 where the concentration of HCL inwater was ramped in second solution 2A during Electrospraying of a 180μM solution 1A of anthracene in acetonitrile. As the HCL concentrationincreased in second solution 2A, the total ES current increased. Themaximum anthracene signal was achieved at approximately 250 nA total EScurrent. Signal response curves 61 and 62 for a 1 μM solution ofhexatyrosine in 1:1 methanol:water versus total ES current are alsoshown in graph 63 of FIG. 7. Curve 61 was generated using a pH gradientwith HCL acid in water run in second solution 2A while Electrosprayingthe above hexatyrosine sample solution 1A at 10 μl/min. For directcomparison, curve 62 is a signal response curve of the same hexatyrosinesample solution 1A sprayed from a conventional Electrospray probe withincreasing concentrations of HCL added directly to sample solution 1A.

PH scans can be conducted during Electrospray ionization to studyprotein and noncovalently bound compound conformations using the newcharged droplet sprayers configured according to the invention. FIG. 8shows the changes in ES-MS spectra acquired during Electrospraying of a1.2 μM aqueous solution 1A of horseheart Myoglobin while running a rapidpH gradient in solution 2A. The concentration of HCL acid in aqueoussolution 2A was ramped using charged droplet sprayer 44 during pneumaticnebulization assisted Electrospray ionization. FIG. 8A shows the ES-MSspectra with a 100% aqueous solution 2A producing a total ES current of33 nA. The signal amplitude is reduced due to a limit in total availablecharge. A high percentage of the myoglobin in the aqueous samplesolution remains in a folded configuration retaining the heme group. Theobserved adduct peaks are due to contaminant species present in theMyoglobin sample purchased from Sigma. As the HCL acid concentration insolution 2A is ramped, increasing the total ES ion current and loweringthe pH in solution 1A, the myoglobin molecule begins to unfold insolution and loses the heme group as shown progressively in FIGS. 8B, 8Cand 8D. In the series of mass spectra acquired in FIG. 7, samplesolution 1A flow was constant with charged species added only throughmembrane 3 of the charged droplet sprayer 44. Charged droplet sprayer 44can be operated by rapidly scanning total ES ion current and/or pH insolution 1A with little or no addition of contamination species tosample solution 1A. Adjustment of conductivity and the composition ofcharged species in solution 2A allows rapid optimization of ES/MSperformance to achieve maximum analyte signal for the same samplesolution. This capability is particularly useful in providing optimalES/MS performance in high throughput and target compound analysis.Changes in confirmations of proteins or non covalently bound compoundsin solution that are observable through shifting multiply charged peakpatterns and losses of non covalently bound groups can be rapidlyscanned to provide additional information when studying protein or noncovalently bound complex structures.

In alternative embodiments to the invention, different types ofmaterials can be used for semipermeable membrane 3 in charged dropletsprayers 4 and 44 to maximize analytical performance for specificapplications using positive or negative polarity Electrosprayionization. FIG. 9 shows a cross section view of the flow channels land2 separated by an alternative membrane 64 configured to facilitateionization of hydrogen gas flowing through channel 2. Hydrogen gas isionized on the surfaces of platinum particles 69 embedded in carbonelectrode supports 68 located in flow channel 2. Semipermeable membrane64 contacts solution 1A in flow channel 1 along membrane surface 65.Membrane 64 can be hydrated by water in solution 1A or by water vaporadded to the hydrogen gas flowing through flow channel 2. Along membranesurface 67 in contact with flow channel 2, carbon electrodes 68 imbeddedwith platinum catalyst particles 69 are bonded to surface 67 ofsemipermeable dielectric membrane 64. Porous carbon fiber mat 70electrically connects carbon electrodes 68 to electrode 5. This carbonsupported platinum catalyst material is well known in fuel celltechnology (Fuel Cell Systems Explained, J. Larminie and A. Dicks, JohnWiley and Sons, 2003, Chapter 4)[13] and is used as a conductive surfaceto ionize hydrogen in such devices. Hydrogen gas is ionized at thesurface of the platinum particles forming protons with electrons removedthrough electrode 5 to power supply 27. The protons or H+ ions passthrough semipermeable membrane 64 into sample solution 1A driven byelectric field 74 sustained during Electrospray ionization. Ion currentpasses along flow channel 1 driven by sample solution flow 72 andelectric field 73. Ion current exits flow channel 1 as charged dropletsforming at exit end 12. The total Electrospray current can be controlledby adjusting the flow rate or concentration of hydrogen gas flow 71passing through flow channel 2. Positive polarity charged droplet sprayis produced from solution 1A using charged droplet sprayer membrane 64with proton transfer through membrane 64 into solution 1A as shown inFIG. 9. Alternatively, negative polarity charged droplet production canbe produced by configuring semipermeable membrane membrane 64 with theappropriate material to produce negative polarity ions from oxygen orother appropriate gas flowing through flow channel 2. Negative polarityions move through semipermeable membrane 64 driven by the Electrosprayelectric field. In an alternative embodiment of charged droplet sprayers4 or 44, electrode 5 can be configured with a platinum surface tocatalyze the ionization of hydrogen gas to form protons that move acrosschannel 2 through membrane 3 into solution 1A driven by the Electrosprayelectric field.

Negative polarity charge droplet sprays are generated by transferringprotons or positive ions across membrane 3 from solution 1A to gas orsolution 2A or by passing negative ions produced in gas or solution 2Ainto solution 1A through membrane 3 driven by the negative polarityElectrospray electric field. Different materials can be used forsemipermeable membrane 3 to selectively transport specific anion speciesor electrons from channel 2 to channel 1 in negative polarity chargeddroplet production. When orifice 24 is configured as a dielectriccapillary orifice into vacuum as described in U.S. Pat. No. 4,542,293incorporated herein by reference, electrode 5 can be operated at groundpotential in both positive and negative ion polarity. In positivepolarity Electrospray ionization, −4,000 V and −5,000 V are appliedtypically applied to electrodes 22 and 23. Voltage polarity is reversedfor negative polarity Electrospray ionization. With close to groundpotential applied to electrode 5 during positive or negativeElectrospray ionization, minimum redox reactions occur in the samplesolution on grounded upstream conductive surfaces in flow channel 1during Electrospray ionization. Preventing redox reactions occurring onconductive surfaces upstream of flow channel 1 minimizes changes insample composition prior to Electrospray ionization. Minimizing changesto sample composition caused by redox reactions in the sample solutionflow path increases Electrospray MS analysis quantitative andqualitative accuracy, consistency and reliability. The electricalcurrent produced from redox reactions upstream of flow channel 1 can bemeasured by closing switch 33 connecting conductive tube 10 with powersupply and current meter 32. The voltage applied to electrode 5 throughpower supply 27 can be adjusted to zero the electrical current producedat tube 10 by neutralizing the electric field upstream of flow channel 1that may cause redox reactions to occur on conductive surfaces. Forexample, a small positive potential above zero volts applied toelectrode 5 during positive polarity Electrospray ionization, minimizesredox reactions from occurring on upstream grounded conductive surfaces.The small positive electrical potential offset applied to electrode 5counters the slightly negative electric field relative to groundextending through flow channel 1 with the above listed kilovoltpotentials applied to electrodes 22 and 24. This results in a neutral orground potential extending upstream from flow channel 1 preventing redoxreactions on grounded upstream conductive surfaces.

Electrospray ion sources that are not configured with a dielectriccapillary orifice into vacuum are typically configured with a conductiveorifice or heated conductive capillary orifice between atmosphericpressure and the first vacuum pumping stage. A conductive orifice intovacuum is typically operated closer to ground potential duringElectrospray ionization. Electrospray ion source configured withconductive orifices into vacuum can be operated with positive andnegative kilovolt potentials applied to electrode 5 during positive andnegative polarity Electrospray ionization respectively. Applyingkilovolt electrical potentials to electrode 5 may result in generationof current on grounded conductive surfaces upstream of flow channel 1due to electrochemical reactions in solution 1A. These upstreamelectrochemical reactions in solution 1A can be avoided by eliminatingor electrically floating conductive surfaces configured upstream of flowchannel 1. It is known in Electrospray operation where redox reactionsoccur on first solution flow channel conductive surfaces, that anion orcation species can be deposited on these conductive surfaces. Whenpositive polarity Electrospray is switched to negative polarityElectrospray, anion species deposited on conductive surfaces can reentersample solution 1A as contamination species. Redox reactions occur onsurfaces external to the first solution 1A flow path in the chargeddroplet sprayer embodiments 4 and 44 shown in FIGS. 1 and 2, avoidingdeposition of contamination species on conductive surfaces in the firstsolution 1A flow path. Operation of charged droplet sprayers 4 or 44also avoids the buildup of deposited species that can ultimately blockflow channels. When stainless steel Electrospray needles are configuredas spray tips in conventional Electrospray operation, metal ions fromthe stainless steel may be produced due to the redox reactions occurringon the inner wall of the Electrospray needle. These metal ions presentin the Electrospray solution produce unwanted contaminant ion peaks inthe acquired mass spectrum. Deplating of metal Electrospray needles orstainless steel conductive surfaces in the sample solution flow pathduring Electrospray operation can be prevented using the charged dropletsprayer embodiments 4 and 44 shown in FIGS. 1 and 2.

A single stable Electrospray Taylor cone can deliver a limited amount ofcharged droplet spray current. Above this limit, the Taylor cone and thecharged droplet production from the Taylor cone will become unstable.Total Electrospray current can be increased by increasing theconductivity in a first or sample solution of 1:1 methanol:water by theaddition of acid (or salts) or through the addition of electrolytes insolution 2A in charged droplet sprayers 4 and 44. A single ElectrosprayTaylor cone can become unstable if total charged droplet spray currentexceeds a value that is a function of solution composition, liquid flowrate, needle and spray tip geometry, solution flow path geometry,electrode geometry and applied voltage. For example, a Taylor coneformed when Electrospraying a methanol:water:acid solution at 5 μl/minmay become unstable between 200 and 300 nanoamps total Electrospraycurrent. FIG. 10A is a diagram of a stable Electrospray Taylor cone 83formed from solution 82 flowing through tube 84 producing evaporatingcharged droplet spray plume 81 moving toward counter electrode 80. Theinitial charged droplets are produced in Electrospray with approximatelyone half the Rayleigh limit of charge. With a constant flow rate ofsolution 82 through tube 84, an increase in total Electrospray currentrequires that an increasing number of droplets are produced with areduced droplet size distribution. An increased number of smaller sizedroplets provides additional total surface area, increasing the chargecarrying capacity of the spray. As described above, the total chargeddroplet current produced from charged droplet sprayer 4 or 44 can beincreased by increasing the conductivity or electrolyte concentration insolution 2A flowing through channel 2. As the charged droplet spraycurrent increases, charged droplet plume 81 fans out due to increasedcharged droplet space charge repulsion. When the charged droplet spraycurrent exceeds the stability limit of a single Electrospray Taylorcone, multiple spray plumes 85 and 86 form from tube 84 exit tip 87 asdiagramed in FIG. 10B. Stable single or multiple charged droplet sprayplumes are produced using charged droplet sprayer 4 or 44 with firstsolution 1A comprising 1:1 methanol:water flowing at 5 μl/min with theaddition of protons to solution 1A through membrane 3. Comparably stablesprays are difficult to achieve with conventional Electrospray apparatususing conductive Electrospray needle tips. Currents exceeding 300nanoamps can be achieved with stable multiple Electrospray plume chargeddroplet spraying of 1:1 methanol:water solutions 1A from single exit tip12 using the charged droplet sprayer embodiments 4 and 44 shown in FIGS.1 and 2 without pneumatic nebulization. To achieve increased totalcharged droplet spray current capacity from charged droplet sprayers 4and 44, multiple spray tips can be configured from flow channel 1 asdiagrammed in FIG. 11.

An alternative embodiment to the invention is diagramed in FIG. 11wherein multiple spray tips are connected to flow channel 100 in chargeddroplet sprayer 88. Solution 100A is introduced into multiple tippedcharged droplet sprayer 88 through tube 89 into channel 100. Channel 100connects to channels 123, 124 and 125 in tubes 92, 93 and 94respectively through low dead volume junction 122 configured indielectric body 108. Solution 100A is Electrosprayed simultaneously fromtube 92 exit tip 112, tube 93 exit tip 113 and tube 94 exit tip 114 withcharged species transferred across membrane 103. Solution or gas 102Aenters through tube 105, flows through flow channel 102 and exitsthrough tube 90. Alternatively, solution or gas 102A can enter throughtube 90 and exit through tube 105. Solution 102A contacts electrode 91and dielectric membrane 103 as it flows through channel 102.Semipermeable dielectric membrane 103 serves the same functions asmembrane 3 described above. All elements and surfaces configured infirst solution 100A flow channel 100 comprise dielectric materials toavoid conducting redox reactions on conductive surfaces in firstsolution 100A flow pathway 100. Alternatively, elements such as tubes92, 93, 94 and 89 may comprise conductive materials but are electricallyfloated during charged droplet spraying to prevent redox reactions fromoccurring on inside channel surfaces. If tube 89 comprises a conductivematerial it may be connected or disconnected from power supply 95 usingswitch 96. Electrical potential is applied to electrode 97 through powersupply 98. Electrode 97 is electrically insulated by dielectric body 108and has no direct contact with solution 100A. Electrode 91, electricallyisolated in dielectric body element 121, is configured to be in directcontact with gas or solution 102A flowing through flow channel 102.

Similar to the single tip charge droplet sprayer embodiments shown inFIGS. 1 and 2, the total Electrospray current produced from the multipletip spray configuration shown in FIG. 11 is a function of the relativeelectrical potentials applied between electrode 91 and counterelectrodes 110 and 111, the compositions of solutions 100A and 102A, theflow rate of solution 100A and the distance between exit tips 112, 113and 114 and counter electrode 110. Electrodes 91, 110 and 111 areconnected to voltage supplies 120, 115 and 116 respectively. Stablesingle plume or multiple plume Electrosprays can be produced from allexit tips simultaneously when operating charged droplet sprayer 88 shownin FIG. 11. The relative position and angles of exit tips 112, 113 and114 can be changed by adjusting mounting bracket 117 joints 118 and 119and by sliding tubes 92, 93 and 94 through mounting bracket 117. Chargeddroplet sprayer 88 shown in FIG. 11 may be configured and operated withone, two or more than three spray tips. The total charged droplet spraycurrent produced from multiple exit Electrospray tips, operating withsingle or multiple stable Electrospray plumes formed at each exit tip,can be adjusted by changing the acid, base, salt, buffer or otherelectrolyte concentration in solution 102A. Total charged dropletElectrospray currents exceeding 1.4 microamps have been achieved with afive spray tip embodiment of charged droplet sprayer 88.

An alternative embodiment to the invention is shown in FIG. 12.Pneumatic nebulization is added to multiple spray tip charged dropletsprayer 88. Flow channel 100 connects to flow channels 123, 124 and 125through low dead volume junction 122 as described above. Tubes 92, 93and 94 are configured with pneumatic nebulizer gas flow assembly 127 toform gas flow annuli 128, 129 and 130 around tubes 92, 93 and 94respectively. Nebulization gas 131 enters nebulizer gas flow assembly127 and exits at outlets 132, 133 and 134 providing gas nebulizationshear forces to aid charge droplet formation at exit tips 112, 113 and114 respectively. Nebulization gas 131 flow rate can be adjusted tooptimize charged droplet production performance for different solution101A compositions or flow rates. Nebulizer gas flow assembly 127 maycomprise a dielectric or conductive material. When nebulizer gas flowassembly 127 comprises a dielectric material, the electric field linespassing through such material will wrap more tightly around exit tips112, 113 and 114 creating a higher electric field at exit tips 112, 113and 114 during Electrospraying. This effective decrease in the exit tipradius of curvature results in a higher local electric field at exittips 112, 113 and 114 for a given relative voltage applied to electrodes91 and counter electrodes 110 and 111. Higher local fields maintained atexit tips 112, 113 and 114 below the onset corona discharge provide moreefficient charging of droplets during charged droplet spraying.Pneumatic nebulization assembly 127 may comprise independentlyadjustable positioning of spray tips 112, 113 and 114.

Multiple second solutions can be separated from a first solution asshown in an alternative embodiment of the invention diagrammed in FIG.13. Charged droplet sprayer 140 comprises dielectric body 144 and threeflow channels 141, 142 and 143. Flow channel 141 is separated from flowchannel 142 by semipermeable dielectric membrane 145 and from channel143 by dielectric membrane 147. Solution or gas 142A flowing throughchannel 142 contacts membrane 145 and electrode 150 connected to powersupply 151. Solution or gas 142A enters and exits flow channel 142through tubes 152 and 153. Solution or gas 143A flowing through channel143 contacts membrane 147 and electrode 154 connected to power supply155. Solution or gas 143A enters and exits flow channel 143 throughtubes 156 and 157. Sample solution 141A enters channel 141 through tube148 and exits through channel 158 of tube 159, forming charged dropletspray 160 from exit tip 161. Charged droplet formation can be aided bynebulizer gas 162 flowing through tube 163 and annulus 164 bounded bythe inner diameter of tube 165 and the outer diameter of tube 159.Nebulizer gas 162 exits at annulus 164 exit 167 surrounding exit tip161. Similar to a two solution charged droplet sprayer as shown in FIGS.1 and 2, the three solution embodiment provides charged species transferthrough both semipermeable membranes 145 and 147 separating solution141A from solutions or gas 142A and 143A respectively. Charged speciestransferring through membranes 145 and 147 during charged dropletspraying are driven by the relative voltages applied between electrodes151 and 155 and counter electrodes 168 and 170. Counter electrodes 168and 170 are connected to power supplies 169 and 171 respectively.

Charged droplet sprayer 140 can be operated with different compositionsfor solutions or gases 142A and 143A. Semipermeable membranes 145 and147 may comprise different materials. The compositions of solutions orgases 142A and 143A and semipermeable membranes 145 and 147 can beoptimized for different applications and operating modes. For examplethe composition of solution or gas 142A and the material used formembrane 145 can be optimized for positive polarity charged dropletproduction. The composition of solution or gas 143A and the compositionof membrane 147 can be optimized for negative polarity charged dropletproduction. Rapid switching between positive and negative polaritydroplet production can be achieved by applying the appropriate voltagesto electrodes 151 and 155 and counter electrodes 168 and 170. Relativeelectrical potentials can be applied between electrodes 151 and 155 toincrease or decrease the total Electrospray current or to change therelative concentration of different charged species in sample solution141A during charged droplet spraying. Two different cation species canbe transferred into solution 141A through two different membranes 145and 147 from two different solutions or gas compositions 142A and 143Aduring positive polarity charged droplet spraying. Alternatively, anionsmay be removed from sample solution 141A through membrane 145 or 147during positive polarity Electrospray ionization. Similarly, in negativepolarity Electrospray, two different anion species can be transferredinto solution 141A through different membrane 145 and 147 compositions.

The composition or concentration of charged species transferred intosample solution 141A can be modified during Electrospraying by changingthe relative voltages applied between electrodes 150 and 154 and/orchanging the composition of solutions 142A and 143A. For example,increasing the voltage applied to electrode 150 and lowering the voltageapplied to electrode 154 during positive polarity charged dropletspraying will increase the rate of cation transfer across membrane 145while decreasing the rate of cation transfer across membrane 147. Therelative amounts or currents of cation or anion species transferredthrough membranes 145 and 147 can be ramped or scanned by changing therelative voltages applied to electrodes 150 and 154 during chargeddroplet spraying. Transfer of cations into solution 141A throughmembrane 145 while simultaneously removing anions from solution 141Athrough an appropriate anion exchange membrane 147 can be performedduring positive polarity charged droplet spraying by applying theappropriate relative electrical potentials to electrodes 151 and 155 andcounter electrodes 168 and 170. Removal of anions during positivepolarity charged droplet spraying may aid in reducing unwanted speciesin solution such as non volatile salts that can contaminate an ionsource during operation. Relative adjustments to anion and cationexchange across membranes 145 and 147 can be conducted in negativepolarity Electrospray by adjusting the relative voltages applied toelectrodes 150 and 154 and counter electrodes 168 and 170. Similar tothe single membrane probe embodiment, ramping or scanning of chargedspecies composition and concentration in solution 141A can be achievedby ramping the concentration of acids, bases, salts, buffers or otherelectrolytes in solutions 142A and 143A during Electrospraying.Adjustments to relative voltages applied to electrodes 150 and 154 andramping the composition of gases or solutions 142A and 143A can beconducted simultaneously during Electrospraying to optimize ES/MSperformance. During operation of charged droplet sprayer 140, includingramping of total Electrospray current and/or ramping of charged speciescomposition in solution 141A, no redox reactions occur on conductivesurfaces in the flow path of solution 141A.

A cross section diagram of an alternative embodiment of the chargeddroplet sprayer is shown in FIG. 14. Charged droplet sprayer assembly174 comprises two first sample solutions 180A and 181A flowing throughflow channels 180 and 181 respectively. Flow channels 180 and 181 areseparated from a third flow channel 182 by dielectric semipermeablemembranes 183 and 184 respectively. Semipermeable membranes 183 and 184comprise materials chosen to pass selected cations or anions asdescribed in the above sections. Electrode 185, configured as a porousmaterial in flow channel 182 yet sealed or solid along external orsealing edge 198, allows solution or gas 182A to move through it whilepassing along channel 182. Solution or gas 182A entering channel 182through tube 187 moves through porous electrode 185 and exits throughtube 188. Alternatively, flow of solution or gas 182A can enter and exitchannel 182 in the opposite direction. Solution 180A enters channel 180through tube 189 and exits through tube 196 at exit tip 190. Solution181A enters channel 181 through tube 191 and exits through tube 195 exittip 192. Electrodes 193 and 194, connected to power supplies 195 and 196respectively, are electrically insulated from flow channels 180 and 181,respectively, by dielectric charged droplet sprayer body 216. Theelectrical potentials applied counter electrodes 197 and 198, connectedto power supplies 199 and 200 respectively and electrode 185 connectedto power supply 203 form an electric field at exit tip 190. Similarly,electrical potentials applied to counter electrodes 204 and 205,connected to power supplies 206 and 207 respectively and electrode 185form an electric field at exit tip 192. The same polarity voltage may beapplied to counter electrodes 197, 198, 204 and 205 whereby the samepolarity charged droplets are sprayed from exit tips 190 and 192 formingcharged droplet sprays 210 and 211, respectively. Alternatively,opposite polarity electrical potentials may be applied to counterelectrodes 197 and 198 compared to electrical potentials applied tocounter electrodes 204 and 205. In this operating mode, positive andnegative polarity charged droplets are sprayed simultaneously from exittips 190 and 192. Charged droplet sprays 210 and 211 may be formed byElectrospraying from exit tips 190 and 192 respectively, or may beformed using pneumatic nebulization in the presence of an electricfield. Nebulizing gas 212 passes through tube 213 and annulus 214,bounded by tubes 215 and 196, exiting at 217 surrounding exit tip 190.Similarly, nebulizing gas 218 passes through tube 219 and annulus 410,bounded by tubes 411 and 195, exiting at 412 surroundng exit tip 192. Aportion of the ions generated from evaporating charged dropletsElectrosprayed from exit tip 190 pass through orifice 201 into vacuumwhere they are mass to charge analyzed by mass to charge analyzer 202.Simultaneously, a portion of the ions generated from evaporating chargeddroplets Electrosprayed from exit tip 192 pass through orifice 208 intovacuum where they are mass to charge analyzed by mass to charge analyzer209.

Charged droplet sprayer assembly 174 allows simultaneous spraying ofopposite polarity charged droplets from two solutions 180A and 181A byapplying the appropriate electrical potentials to electrodes 185, 198,197, 204 and 205 as described above. Flow channels 180 and 181 can beconfigured so that no connected conductive surfaces are in contact withsolutions 180A or 181A during Electrospraying. With no conductivesurfaces in contact with solutions 180 and 181, all charge species addedto or removed from these solutions during charged droplet spraying, passthrough membranes 183 and 184, respectively. Solutions 180A and 181A maycomprise the same solution from a common source or different solutions.The electrical potentials applied to electrodes 193 and 194 may be setto modify the current flowing through membranes 183 and 184respectively, however, the electric field established at Electrospraytips 190 and 192 provide the dominant driving force in determining thetotal Electrospray current generated at charged droplet sprayer exittips 190 and 192. The electrical current carried by charged speciespassing through membranes 183 and 184 and through channels 180 and 181respectively, can be increased by increasing the concentration of themembrane permeable cations or anions in solution 182A. The same oropposite polarity charged droplets may be sprayed simultaneously fromexit tips 190 and 192 by applying the same or opposite polarityelectrical potentials but not necessarily the same voltage amplitudes tocounter electrode sets 197 with 198 and 204 with 205. The voltage valuesapplied to counter electrode sets 197 with 198 and 204 with 205 relativeto the potential applied to electrode 185 can be individually adjustedto optimize charged droplet spray currents, independently, at exit tips190 and 192. Charged droplet sprays from exit tips 190 and 192 can beturned on or off independently, or operated simultaneously, by applyingthe appropriate voltages. The relative orientation of exit tips 190 and192 may be optimized for any given application or instrument geometry.For example, simultaneous positive and negative polarity charged dropletspraying from the same solution allows the simultaneous analysis ofpositive and negative ions produced from the drying charged droplets bytwo mass to charge analyzers 202 and 209.

The electrical current produced from redox reactions occurring in thesecond or non sample solution flow channels of the charged dropletsprayer embodiments shown above during charged droplet spraying aredetermined by the relative electrode and counter electrode potentialsand geometries, the composition of the solutions present in the flowchannels and the first solution flow rate. Electrical potential appliedto the insulated electrodes configured adjacent to the first solutionflow channel has a relatively small influence on the droplet spraycurrent produced. A more effective placement of an electricallyinsulated electrode configured to allow adjustment of the totalElectrospray current during operation is shown in FIG. 15. Chargeddroplet sprayer 220 is configured with sample solution flow channels 221and second gas or solution flow channel 222 in dielectric body 248.Electrode 223 is in contact with solution 222A in flow channel 222.Electrode 224 is insulated by dielectric body 248 from contact withsolution 221A in flow channel 221. Flow channels 221 and 222 areseparated by semipermeable dielectric membrane 225 as has been describedabove. Electrically insulated porous electrode 228 with solid sealingedges 229 is configured in flow channel 221, either flush with thesurface of or incorporated into membrane 225. One embodiment ofelectrode 228 is a porous grid of PTFE (Teflon) coated wire where thePTFE coating is bonded to the surface membrane 225 in contact withsolution 221A. The insulating layer surrounding the electrode wire ininsulated porous electrode 228 prevents contact between the electrodeand solution 221A and prevents the neutralization of charged speciesbeing transferred through membrane 225 between flow channels 222 and221.

Solution 221A enters through tube 233, flows through channel 221 andchannel 235 of tube 234 and exits at exit tip 237 forming chargeddroplet spray 238. Charged droplets are produced by Electrospraying fromexit tip 237 or are formed by pneumatic nebulization in the presence ofan electric field as solution 221A leaves exit tip 237. Nebulization gas250 enters through tube 239 and flows through annulus 241 bounded bytubes 240 and 234 and exits at 242 surrounding exit tip 237. Solution orgas 222A can flow in either direction through channel 222 enteringand/or exiting from tubes 231 and 232. Solution 221A is in contact withthe insulation of insulated porous electrode 228 and membrane 225 as itflows through channel 221. Electrical potentials are applied toelectrodes 223, 228 and 224 through power supplies 226, 230 and 227respectively. Electrical potentials are applied to counter electrodes243 and 244 through power supplies 246 and 245 respectively. In theembodiment of charged droplet sprayer 220 shown, no electricallyconnected conductive surface is in contact with solution 221A duringcharged droplet spraying. Tube 234 is configured as either dielectricmaterial such as fused silica or is electrically floated or isolatedwhen comprising a conductive material. The thickness and porosity ofporous insulated electrode 228 is configured to allow sufficientelectric field penetration from channel 221 into membrane 225, tofacilitate the transfer of charge from the surface of membrane 225 intoflow channel 221. The electric field penetration into membrane 225 fromchannel 221 does not penetrate significantly into channel 222. Therelative potentials applied to insulated porous electrode 228 andelectrode 223 can be set to reduce or enhance the electric field presentin flow channel 221. Adjustment of the voltage applied to porousinsulated electrode 228 serves to modify the electric field formedthrough solution 221A between electrode 223 and counter electrodes 243and 224 during charged droplet spraying. The electrical potentialapplied to electrically insulated electrode 228 can be set to reduce orincrease the charged droplet spray current produced for a given solution221A and solution or gas 222A, electrode geometry, flow rate of solution221A and relative electrical potentials set on electrodes 223 and 224and counter electrodes 243 and 244.

The electric field formed between electrode 223 and electricallyinsulated electrode 228 will influence the electrical current generatedat electrode 223 through electrolytic or other redox reactions occurringin flow channel 222. In the embodiment of charged droplet sprayer 220shown, charge species passing through membrane 225 are transferred intosolution 221A through the gaps in porous insulated electrode 228 duringcharged droplet spraying. FIG. 16 shows a cross section diagram of flowchannels 221 and 222. Solution or gas 222A is in contact with electrode223 and semipermeable dielectric membrane 225 as it flows throughchannel 222. Solution 221A is in contact with dielectric body 248,dielectric membrane 225, and electrical insulation 253 surroundingconductive elements 254 of electrically insulated porous electrodeassembly 228. FIG. 16 shows an example of proton (H⁺) transfer throughmembrane 205 from solution 222A into solution 221A during positivepolarity Electrospray. In one embodiment of the invention, membrane 225is configured as Nafion material as described above. The strength ofelectric field 251 or E₁ is determined by the electrical potentialsapplied between electrode 223 and insulated electrode 228 through powersupplies 226 and 230 respectively combined with the electric field 252or E₂ maintained at exit tip 237 by voltages applied to counterelectrodes 243 and 244. A voltage drop occurs along flow channels 235and 221 through solution 221A with electrical field penetration throughinsulated electrode 228 and semipermeable dielectric membrane 225. Therelative electrical potentials applied between electrode 223 and 228 canbe set to strengthen or weaken electric field E₁ through membrane 225and across channel 222. H+ ions produced at electrode 223 aretransferred through membrane 225 driven initially by electric field E₁and subsequently transferred from membrane 225 into flow channel 221 bythe penetration of field 252 or E₂ into membrane 225 through the gapsbetween insulated electrode elements 253 and 254 of electricallyinsulated electrode assembly 228. Increasing electric field E₁ increasesthe total Electrospray current. The total Electrospray current producedfrom solution 221A can be rapidly adjusted or scanned duringElectrospray ionization by changing or ramping the composition ofsolution 222A and/or by changing the relative voltages applied toelectrode 223 and 228. The embodiment of the invention diagrammed inFIGS. 15 and 16 allows the rapid optimization of charged droplet spraycurrent for a given application with a voltage adjustment applied toelectrode 223 for a given solution 221A composition. Electrospraycurrent may be adjusted to optimize performance in the time frame of aneluting liquid chromatography peak. Voltage applied to electrode 223 canbe adjusted based on data dependent software feedback during LC/MSoperation. A portion of the ions produced from Electrospray 238 passthrough the orifice in electrode 244 into vacuum where they are mass tocharge analyzed. In alternative embodiments of the invention,electrically insulated porous electrode assembly 228 can be attached toa surface of or incorporated into semipermeable dielectric membrane 225or positioned between layers of a multiple layer semipermeable membrane225 configuration.

Increased flexibility in charged droplet spray operation can be achievedby configuring one first solution flow channel separated from two secondsolution flow channels by two insulated porous electrode assembliespositioned adjacent to two semipermeable dielectric membranes asdiagrammed in FIG. 17. First solution 261A enters flow channel 261through tube 290, traverses flow channel 261 configured in dielectricbody 270 and exits through channel 287 in tube 288 at exit tip 275.Second solution 262A entering and exiting flow channel 262 through tubes292 and 293 contacts electrode 269 and semipermeable dielectric membrane267. Membrane 267 and insulated porous electrode assembly 268 separateflow channels 262 and 261. Semipermeable dielectric membrane 265 andadjacent insulated porous electrode 266 separate flow channels 263 and261. Second solution 263A enters and exits flow channel 263 throughtubes 294 and 295. Electrode 264 and membrane 265 are in contact withsolution 263A as it flows through channel 263. Charged droplet spray 291is formed from solution 261A at exit tip 275 by unassisted Electrosprayor Electrospray with pneumatic nebulization assist. Nebulizing gas 284flows through tube 283 and annulus 286 bounded by tubes 285 and 288exiting at 289 surrounding exit tip 275. Charged droplet sprayer 260 isconfigured such that no redox reactions occur on surfaces in the flowpath of first solution 261A. The total Electrospray current is providedby charged species transferred through membranes 267 and/or 265 eitherinto or out of solution 261A during charged droplet spraying. Thecharged droplet spray current depends on the relative potentials appliedto electrodes 269, 264, 268 and 266 through power supplies 271, 272, 274and 273, respectively and applied to counter electrodes 276 and 278through power supplies 279 and 280, respectively.

As is the case with the two flow channel charged droplet sprayerembodiment shown in FIG. 15 and described above, the current of chargedspecies passing through semipermeable dielectric membrane 267 in chargeddroplet sprayer 260 can be adjusted by changing the relative electricalpotentials applied between electrode 269 and electrically insulatedporous electrode 268. Similarly, the current of charged species passingthrough semipermeable dielectric membrane 265 can be adjusted bychanging the relative electrical potentials applied between electrode264 and electrically insulated porous electrode 266. Charged dropletsprayer assembly 260 comprising three flow channels provides additionalflexibility in optimizing charged droplet production for specificapplications. The flexibility in operating modes described for threeflow channel charge droplet sprayer 140 diagrammed in FIG. 13 applies tothe operation of charged droplet sprayer 260. The addition of twoelectrically insulated porous electrode assemblies 268 and 266configured in flow channel 261 adjacent to semipermeable membranes 267and 266 respectively allows the adjustment of electrical current carriedby charged species transferred through membranes 267 and 266. Thiscompliments control of the Electrospray charged droplet productionprocess provided by changing the composition of solutions 262A or 263A.Adjustment of charged droplet spray 291 total Electrospray current canbe achieved during Electrospray ionization by changing electricalpotentials applied to insulated porous electrode assemblies 266 and 268configured in charged droplet sprayer 260.

Conventional Electrospray probes configured with conductive surfaces inthe sample solution flow path may deposit species on the sample solutionflow channel conductive surfaces due to redox reactions duringElectrospray ionization. The species that deposit on conductive surfacesdue to redox reactions in one ion polarity Electrospray operating modemay deplate and reenter the sample solution through the reverse redoxreaction in the reverse ion polarity Electrospray operating mode. Theredesolved species reentering the sample solution when the Electrospraypolarity is reversed can produce unwanted contamination or interferencepeaks in the acquired mass spectrum or can modify the true analytesignal due to charge competition or reactions in solution. Theembodiments of the invention described above add or remove chargespecies from the first solution flow through semipermeable membranes,minimizing or preventing deposition of material occurring on conductivesurfaces in the first or sample solution flow path. The embodiments ofthe invention described also provide a means to control the chargeddroplet spray current for the same sample solution to optimize chargeddroplet spraying for on line LC/MS or offline analytical applications.The charged droplet sprayers configured according to the invention mayalso be interfaced to ion mobility separation devices including but notlimited to High Field Asymetric Waveform Ion Mobility Spectrometry(FAIMS) configured in atmospheric pressure ion source assemblies.

An alternative embodiment to the invention, diagrammed in FIG. 18,provides means for separation of charge in the first solution duringcharged droplet spraying while avoiding redox reactions occurring onconductive surfaces in the first solution flow path. Limited control ofthe total charged droplet spray current can be achieved using chargeddroplet sprayer 300 shown in FIG. 18 without modifying the compositionof first solution 301A. Anions and cations present in first solution301A separate into two solution flow paths during simultaneous positiveand negative charged droplet spraying. Dual exit charged droplet sprayer300 comprises dielectric body 308 with first flow channel 301. Solution301A enters flow channel 301 through tube 307 and bifurcates throughjunction 331 into flow channels 302 and 303. Solution 303A flowingthrough channel 303 in tube 305 exits at exit tip 320 forming chargeddroplet spray 321. Solution 302A flowing through channel 302 in tube 304exits at exit tip 318 forming charged droplet spray 319. Electricalpotentials are applied to counter electrodes 314 and 315 connected topower supplies 316 and 317, respectively. Electrical potentials areapplied to counter electrodes 310 and 311 connected to power supplies313 and 312, respectively. The electrical potentials applied toelectrodes 314 and 315 relative to the electrical potentials applied toelectrodes 310 and 311 are set at an amplitude sufficient to maintainopposite polarity Electrospray from exit tips 320 and 318. No connectedconductive surfaces are configured in the first solution flow path incharged droplet sprayer 300 minimizing the occurrence of redox reactionson such surfaces during charged droplet spraying. Tubes 304 and 305 maybe configured as dielectric material such as fused silica or PEEK or maycomprise conductive material such as stainless steel but areelectrically floated through connection to dielectric body 308.Depending on the relative electrical polarity applied, counterelectrodes 310 and 311 may either provide or accept electronscomplimented by the acceptance or providing of electrons by counterelectrodes 314 and 315 to complete the electrical circuit duringsimultaneous opposite polarity charged droplet spraying.

For example, when −3,000 volts (V) is applied to counter electrode 310,−3,500 V applied to capillary entrance and counter electrode 311, +3,000V applied to counter electrode 314 and +3,500 V applied to capillaryentrance and counter electrode 315, positive polarity charged dropletspray 319 occurs at exit tip 318 and negative polarity charged dropletspray 321 occurs at exit tip 320. In this example, electrons aresupplied through counter electrodes 310 and 311 and are deposited oraccepted on counter electrodes 314 and 315 to complete the electricalcircuit. With positive voltages applied to counter electrodes 310 and311 and equal voltage amplitudes of opposite polarity applied to counterelectrodes 314 and 315, the electric field in flow channel 301 is nearground potential. The relative voltages applied to counter electrodes310 with 311 and 314 with 315 can be adjusted to provide a neutralelectric field relative to ground potential in flow channel 301 tominimize the occurrence of redox reactions on the surfaces of upstreamflow channel conductive elements. This operating mode allows tube 307 tobe connected to a grounded pump or fluid reservoir with no electricalpotential present in solution 301A to cause redox reactions at anygrounded conductive pump, transfer line or fluid reservoir surface.Alternatively, electrodes 310 and 311 can be operated near groundpotential with +6,000 V and +6,500 V applied to electrodes 314 and 315respectively to achieve positive polarity charged droplet spray 319 fromexit tip 318. In the former case with approximately equal but oppositepolarity electrical potential applied to the counter electrodes sets theelectrical potential of the first solution in channel 301 is effectivelyground or zero volts. In the latter operating mode, the relativepotential of solution 301A is approximately +3000V. In this case aconnection to a grounded LC pump through tube 307 may result in redoxreactions on conductive grounded pump surfaces in contact with solution301A. Such redox reactions can be reduced by configuring a highlyresistive flow such as a fused silica packed LC column path between theLC pump and flow channel 301.

Charged droplet sprayer 300 can be operated with unassisted Electrosprayor Electrospray with pneumatic nebulization assist at exit tips 318 and320. Nebulization gas 322 enters channel 323, passes through annulus 324bounded by tubes 304 and 332 exiting at 325 surrounding exit tip 318.Similarly, nebulization gas 327 enters channel 328, passes throughannulus 329 bounded by tubes 305 and 333 exiting at 330 surrounding exittip 320. Pneumatic nebulization can be turned on or off selectively forone or both spray tips during charged droplet spraying. The relativeliquid flow rates through channels 302 and 303 can be adjusted byapplying different forces such as nebulization gas flow differentiallyat exit tips 318 and 320 or modifying the length or inner diameter oftubes 304 and 305. During simultaneous positive and negative chargeddroplet spraying, solutions 302A and 303A have a net charge of oppositepolarity. Anion and cation species are deposited on counter electrodespray surfaces or swept into respective capillary orifices duringcharged droplet spraying. Anions and cations are not deposited oncharged droplet sprayer 300 internal flow channel surfaces through redoxreactions during simultaneous spraying of positive and negative polaritycharged droplets. Positive and negative polarity ions are producedsimultaneously from evaporating charged droplet sprays 319 and 321moving against counter current drying gas flows 334 and 335respectively. A portion of the ion population produced is swept throughcapillary bores 336 and 337 into vacuum where positive and negative ionsare mass to charge analyzed with a single or separate mass to chargeanalyzers 338 and 339 respectively.

The embodiment of the invention as shown in FIG. 18 can be simplified,while retaining performance features and increasing control of thecharged droplet spray process, if simultaneous positive and negativecharged droplet production is not required. A diagram of an alternativeembodiment of the invention is shown in FIGS. 19A, 19B and 19C. Counterelectrodes 314 and 315 as diagrammed in FIG. 18 are replaced byrepositioned counter electrode 341 connected to power supply 342.Countercurrent electrode 341 is positioned relative to exit tip 320 suchthat solution 303A flowing through channel 303 of dielectric tube 305exits at exit tip 320 making contact with counter electrode 341 throughliquid connection 340 as shown in FIG. 19B. During charged dropletspraying from exit tip 318, electrical current flows to counterelectrode 341 and power supply 342 through the liquid connection 340.When operating charged droplet sprayer 348, the electrical circuitformed between power supplies 312 and 313 and power supply 342 iscompleted through charged droplet spray 319 from solution 302A directedto counter electrodes 310 and 311 and through liquid connection 340maintained between solution 303A and counter electrode 341.

Liquid connection 341 eliminates the need to optimize and balance twoopposite polarity sprays simultaneously and provides a more refinedcontrol of the electric field applied at exit tip 320. Anions or cationsneutralized on electrode 341 can be readily washed off as required, evenduring charged droplet spraying, or retained for additional analysis.Electrode 341 can be moved during charged droplet spraying to spatiallyseparate species deposited at different times during MS^(n) or LC/MS^(n)analysis. Such deposited species can be reanalyzed subsequent to theanalysis in which deposition occurred. When positive polarity chargeddroplet spraying of solution 301A containing nonvolatile salts isconducted, anions of the nonvolatile salts are deposited on electrode341 during spraying. This deposition of nonvolatile species reduces theamount of contamination species deposited on counter electrodes 310 and311 from positive charged droplet spray 319. Although solution 301A issplit into 2 flows 302A and 303A flowing through channels 302 and 303respectively, a large percentage of positive ions of interest will bedirected to the positive spray flow channel minimizing positive ionsignal loss in mass to charged analysis during positive polarity chargeddroplet spraying. Similarly, during negative polarity charged dropletspraying, negative ions in solution will be directed into the negativepolarity flow path minimizing any reduction of negative ion signal inmass to charge analysis. The flow rate of solution 303A through flowchannel 303 can be minimized by reducing the diameter of channel 303 orincreasing its length while minimizing the effect on electrical current.Electrospray operation with no flow through channel 303 can be achievedwith the alternative embodiment of the invention diagrammed in FIG. 19C.Charged droplet sprayer 348 dielectric tube 305 and channel 303 is shownas dielectric tube 343 and channel 347 respectively in FIG. 19C. Powersupply 342 connects to solution 303A through the conductive surface 345of electrode 344. Electrical current flows through electrode 344 topower supply 342 during Electrospraying with redox reactions occurringon conductive surface 345 displaced from the solution flow path 302. Inthe embodiment shown in FIG. 19C, all sample solution flow traversesflow channel 302 with only charged species moving through channel 347 toelectrode 344 during Electrospraying. Electrode 344 can be removed andcleaned to prevent carryover of contamination species when switchingpolarity in Electrospray ionization. Alternatively, rapid deplating ofcontamination species can be achieved from surface 345 when switchingpolarity in charged droplet spray compared with conventionalElectrospray probe geometries, reducing the flushing time betweenopposite polarity Electrospray MS analysis.

Dual output charged droplet sprayers as diagrammed in FIGS. 18 and 19can be configured with semipermeable dielectric charge transfermembranes as described for FIGS. 1 through 17 above. The combinedconfiguration of both charged droplet sprayer embodiments shown in FIGS.1 and 18 or 19 provides the operation and performance advantages andimprovements of both individual charged droplet sprayer embodiments. Analternative embodiment to a combined charged droplet sprayer comprisesthe introduction of first and second solutions into a common flowchannel without separation by a semipermeable membrane is diagrammed inFIG. 20. Similar to the dual polarity charged droplet sprayer embodimentdiagrammed in FIG. 18, charged droplet sprayer 350 with dielectric body370 comprises opposite polarity Electrospray or pneumatic nebulizationcharged droplet sprayers. Positive and negative polarity chargeddroplets are simultaneously sprayed from exit tips 360 and 361 duringoperation. Alternatively, exit tip 360 may be configured with arepositioned counter electrode 363 to form a liquid connection withsolution 357A flowing through channel 357 of dielectric tube 359 similarto liquid connection 340 shown in FIG. 19. First or sample solution 353Ais introduced into flow channel 351 through tube 354 and channel 353.Second solution 352A is introduced into flow channel 351 through tube355 and channel 352. Solutions 352A and 353A may mix or may have minimummixing while flowing through channel 351 depending on relativeconcentrations of solution components, relative flow rates, and theinfluence of the electric field applied through flow channels 356 and357. The introduction of solution 352A allows the addition of chemicalspecies required to optimize the performance of the opposite polaritycharged droplet sprays for component sample species in solution 353A forin a given ES/MS^(n) analytical application. For example, cations may beadded through an acid containing solution 352A to an aqueous firstsolution 353A in flow channel 351 during positive polarity chargeddroplet spraying from exit tip 361. Charge separation occurs in mixed orlayered solution 351A as solution 351A flow bifurcates into flowchannels 356 and 357.

As an example of one operating mode, consider positive polarity chargeddroplet spraying from exit tip 361 and negative polarity charged dropletspraying from exit tip 360. Positive electrical potentials applied tocounter electrodes 363 and 364 through power supplies 366 and 365respectively are of equal amplitude but opposite polarity from thenegative electrical potentials applied to counter electrodes 367 and 368through power supplies 369 and 370 respectively. No connected conductivesurfaces are present in the solution flow paths of charged dropletsprayer 350 so redox reactions occurring on flow channel surfaces duringcharged droplet spraying are minimized. Positive charged species insolution 351A will move into channel 356 and negative charged specieswill move into channel 357 during charged droplet spraying. Theembodiment of the invention diagrammed in FIG. 20 allows theintroduction of desired chemical species into the first solution flowand provides separation of charged species in solution of oppositepolarity prior to spraying. Variables such as second solutioncomposition and flow rate and relative electrical potentials applied tocounter electrodes can be adjusted to optimize charge droplet sprayingperformance for specific applications. Operation with pneumaticnebulization of charged liquid droplets from Electrospray tips 360 and361 is achieved by turning on nebulization gas flows 363 and 362respectively

In an alternative embodiment of the invention, diagrammed in FIG. 21,the mixing of a sample solution with a second solution is minimizedwhile retaining the ability to add charged species to or remove chargespecies from the first solution flow during charged droplet spraying.Charged droplet sprayer 380 comprises dielectric body 408 and twosolution inlets and two outlets. First sample solution 381A entersthrough channel 381 in tube 384 and passes through junction 387 becomingsolution 389A. Solution 389A passes through channel 389 in tube 388,exiting at exit tip 390 forming charged droplet spray 407. Secondsolution 383A enters through tube 383, passes through channel 382 andbifurcates into flow channels 386 and 385. Flow channel 385 connectswith flow channel 392 in tube 391. Solution 392A passing through flowchannel 392 exits at exit tip 393 making electrical contact throughliquid connection 394 to counter electrode 395 connected to power supply396. Counter electrodes 397 and 398 are connected to power supplies 399and 400 respectively. Charged species generated in unassistedElectrospray or Electrospray with pneumatic nebulization assist plume407 impinging on counter electrodes 397 and 398 and passing throughcapillary bore 406 complete the electrical circuit with counterelectrode 395 through liquid connection 394 as has been previouslydescribed for charged droplet sprayer 348 diagrammed in FIG. 19.Separation of charged species in solutions 381A and/or 382A occurs atjunction 387 or in flow channel 382 with opposite polarity chargepassing into flow channels 386 and 385. The flow rate of solution 383Aand the flow resistance of channel 392 can be adjusted to determine thenet flow and direction of flow of solution 386A in channel 386.Alternatively, the electrical contact with solution 392A can be madewith a zero flow junction as diagrammed in FIG. 19C. During operation ofthis embodiment, the flow rate and direction of flow through channel 386matches the flow rate and direction of flow of second solution 383A.

When spraying positive polarity charged droplets from exit tip 390,positive charge species can move with solution 386A flow through channel386 adding to first solution 381A at junction 387. Alternatively, duringpositive polarity charged droplet spraying, negative ion speciesseparating from positive polarity ion species in solution 381A move fromsolution 381A into channel 386 at junction 387. The movement of negativecharge species into flow channel 386 can occur with net solution flow ineither direction in flow channel 386. Similarly, when spraying negativepolarity charge droplets from exit tip 390, negative charged species canbe added to solution 381A at junction 387 or positive charged speciescan be removed from solution 381A at junction 387. For either positiveor negative polarity charged droplet spraying, the flow rate anddirection of solution flow through channel 386 can be controlled by theadjusting the flow rate and direction of flow of solution 383A inchannel 382 for a given flow channel 385 and 392 geometry. The geometryof channels 386 and 385 can be modified to optimize solution flow andcharged species movement into or out of first solution 381A. Forexample, channel 386 and 385 can be configured as a single straightchannel with a tee into channel 382 minimizing channel length to reducedead volume and solution electrical and fluid flow resistance. Solution389A can be Electrosprayed from exit tip 390 with or without pneumaticnebulization assist. Nebulizer gas 401 flowing through channel 402 andannulus 403 bounded by tubes 404 and 388 exits at 405 surrounding exittip 390. Electrical potentials applied to counter electrodes 397 and 398through power supplies 399 and 400 respectively form an electric fieldat exit tip 390. The electrical potential applied to counter electrode395 through power supply 396 contacts solution 392A through liquidconnection 394. The occurrence of redox reactions on dielectric orelectrically isolated flow channel surfaces in charged droplet sprayerassembly 380 is minimized during charged droplet spraying. Total chargeddroplet spray current leaving exit tip 390 is matched by electricalcurrent flowing through exit tip 393 and through liquid connection 394to electrode 395. The field strength at exit tip 390, solutioncompositions and flow rates, flow channel geometries and the voltageapplied to counter electrode 395 relative to counter electrodes 397 and398 will determine the total charged droplet spray current leaving exittip 390. Flow rates and the composition of solutions 381A and 383A, thevoltages applied to electrodes 395, 387 and 398 and nebulization gas 401flow rate can be adjusted to optimize charged droplet spray 407 for agiven application. A portion of the ions produced from evaporatingcharged droplet spray 407 are directed through capillary orifice 406into vacuum where they are mass to analyzed by mass to charge analyzer408.

Although flow channels, tubes, junctions and annulus regions are shownin diagrams configured as both integrated and discrete elements, thesestructures and elements can be configured in fully integrated devicesand microfabricated devices to minimize dead volume and to optimize flowchannel geometry. The charged droplet sprayer embodiments of theinvention described above or combinations of such embodiments, producecharged droplet spray currents where all or a portion of such spraycurrent is generated by redox reactions occurring on surfaces externalto the first solution flow path. The total Electrospray current can beadjusted using embodiments of the invention without modifying the inputcomposition of the first solution. Small diameter channels can beconfigured to supply charged species in nanospray devices for firstsolution flow rates less than 1 ul/min. Calibration components orreactants can be added to first solution flows from second solutionsthrough specifically configured selective membranes or flow junctions.Combinations of the embodiments shown in FIGS. 1 through 21 above can beconfigured to utilize the control and performance advantages of eachcharged droplet sprayer embodiment. The charged droplet sprayerembodiments described herein can be configured and operated to optimizeperformance for applications ranging from ion sources in massspectrometers to aerosol generators to painting. Alternative geometriesof the embodiments diagrammed can be configured with variations on theelements described herein. Using the embodiments of the inventions orcombinations of embodiments of charged droplet sprayer devicesconfigured according to the invention, charged droplet spraying may beconducted whereby the total charged droplet spray current generated isgreater than the electrical current occurring due to redox reactions onconductive surfaces in the first solution flow channel. The ratio of thetotal charged droplet spray current generated from redox reactionsoccurring on surfaces external to versus internal to the first solutionflow path can be adjusted using embodiments or combinations ofembodiments of the invention. Ultrasonic nebulization, alternativeconfigurations of pneumatic nebulizers or alternative configurations ofcounter electrodes can be incorporated as alternative embodiments of theinvention.

The invention can be operated to conduct conductivity or pH scans bychanging composition of the second solution, changing the flow rate ofthe first solution for a given second solution composition or changingthe relative potentials applied to selected electrodes as describedabove. Conductivity or pH scanning can be conducted during Electrosprayionization with or without a semipermeable membrane separating thesample solution and second solution. Rapid pH or conductivity scanningcan be conduced during the elution time of a liquid chromatography peakthrough preprogrammed or data dependent control. Scanning pH allows theoptimization of ion signal for sample molecules that have different pIvalues in a sample solution. Multiple membrane interfaces between samplesolutions and second solutions can be configured according to theinvention in parallel or in a serial arrangement in the sample solutionflow paths. Membranes of different thickness and compositions and layersof membranes comprising the same or different materials can beconfigured in charged droplet sprayers configured and operated accordingto the invention.

Although the present invention has been described in accordance with theembodiments shown, one of ordinary skill in the art will recognize thatthere can be variations to the embodiments and such variations wouldfall within the spirit and scope of the present invention.

1. An apparatus for producing charged droplets comprising: a. a firstsolution flow channel with at least one exit end, b. a first solution insaid first solution flow channel, c. at least one second solution flowchannel, d. at least one second solution in said at least one secondsolution flow channel, e. said first and said at least one secondsolution and said first and said at least one second flow channel areseparated by at least one membrane; and f. means for producing a chargeddroplet spray at said exit end of said first solution flow channelwhereby at least a portion of the total charged droplet spray current istransferred through said membrane.
 2. An apparatus for producing chargeddroplets comprising: a. a first solution flow channel with at least oneexit end, b. a first solution in said first solution flow channel, c. atleast one second solution flow channel, d. at least one second solutionin said at least one second solution flow channel, e. said first andsaid at least one second solution and said first and said at least onesecond flow channel are separated by at least one membrane; f. means forproducing a charged droplet spray at said exit end of said firstsolution flow channel whereby at least a portion of the total chargeddroplet spray current is transferred through said membrane; and g. meansfor changing the composition of said second solution during said chargeddroplet production.
 3. An apparatus for producing charged dropletscomprising: a. a first solution flow channel configured with at leastone exit end, b. a first solution flow in said first solution flowchannel c. means for generating one electric field at said at least oneexit end, and d. means for forming a charged droplet spray of said firstsolution from at least one exit end, whereby the current produced bysaid charged droplet spray is greater than the current produced fromredox reactions occurring on conductive surfaces in said first solutionflow channel.
 4. An apparatus for producing charged droplets comprising:a. a first solution flow channel configured with at least one exit end,b. a first solution flow in said first solution flow channel, c. meansfor generating an electric field at said at least one exit end, and d.forming a charged droplet spray of said first solution from at least oneexit end, with no redox reactions occurring on conductive surfaces insaid first solution flow channel.
 5. An apparatus according to claims1-4 whereby said first solution flow channel is configured with at leasttwo exit ends.
 6. An apparatus according to claims 1-4 whereby chargeddroplets of the same polarity are sprayed from at least two of said exitends.
 7. An apparatus according to claims 1-4 whereby charged dropletsof opposite polarity are sprayed from at least two exit ends.
 8. Anapparatus according to claims 1-4 whereby at one of said at least twoexit ends is positioned such that solution leaving said at least oneexit lens forms an electrical contact with a counter electrode.
 9. Anapparatus according to claims 1-4 whereby an insulated porous electrodeis positioned in said first solution flow channel adjacent to saidmembrane.
 10. An apparatus according to claims 1-4 wherein said membranecomprises a semipermeable membrane.
 11. A method for producing chargeddroplets comprising: a. utilizing an apparatus comprising a firstsolution flowing through a first solution flow channel with at least oneexit end having an electric field present at least one exit end and b.spraying charged droplets from said at least one exit end whereby thecurrent produced by said charged droplet spray is greater than thecurrent produced from redox reactions occurring on surfaces in saidfirst solution flow channel.
 12. A method for producing charged dropletscomprising: a. utilizing an apparatus comprising a first solutionflowing through a first flow channel with at least one exit end havingan electric field present at least one exit end and at least one secondsolution flowing through at least one second flow channel whereby saidfirst and said at least one second solution and said first and said atleast one second flow channel are separated by at least one membrane; b.transferring charged species through said at least one membrane forminga current through said at least one membrane; and c. spraying chargeddroplets from said at least one exit tip whereby said charged speciescurrent through said at least one membrane comprises at least a portionof said total charged droplet spray current.
 13. A method for producingcharged droplets according to claim 12, comprising the further step ofchanging said charged droplet spray current by changing the compositionof said second solution.
 14. A method for producing charged dropletsaccording to claim 12 or 13, comprising the further step of changing thepH of said first solution during charged droplet spraying by changingthe composition of said second solution.
 15. A method for producingcharged droplets comprising: a. utilizing an apparatus comprising afirst solution and a first solution flow channel with at least one exitend and a second solution and a second solution flow channel wherebysaid first and said second solution flow channels form a junction; b.transferring charged species through said junction forming a currentthrough said junction; and c. spraying charged droplets from said atleast one exit tip whereby said current through said junction comprisesat least a portion of said charged droplet spray current.
 16. A methodfor producing charged droplets comprising: a. utilizing an apparatuscomprising a first solution and a first and second flow channel witheach said flow channel comprising at exit ends and said second flowchannel forming a junction with said first flow channel whereby saidexit end of said second channel is positioned such that said firstsolution leaving said second channel exit lens an electrical contactwith an electrode; b. spraying charged droplets from said exit end ofsaid first flow channel in the presence of an electric field; and c.adjusting the electrical potential applied to said electrode whereby atleast a portion of the charged droplet spray current is supplied fromsaid electrode through said first solution.